Re: [PR] KAFKA-15778 & KAFKA-15779: Implement metrics manager (KIP-714) [kafka]

[via GitHub]

junrao commented on code in PR #14699:
URL: https://github.com/apache/kafka/pull/14699#discussion_r1391737904


##########
clients/src/main/java/org/apache/kafka/common/requests/PushTelemetryRequest.java:
##########
@@ -60,17 +62,39 @@ public PushTelemetryRequest(PushTelemetryRequestData data, 
short version) {
 
     @Override
     public PushTelemetryResponse getErrorResponse(int throttleTimeMs, 
Throwable e) {
-        PushTelemetryResponseData responseData = new 
PushTelemetryResponseData()
-                .setErrorCode(Errors.forException(e).code())
-                .setThrottleTimeMs(throttleTimeMs);
-        return new PushTelemetryResponse(responseData);
+        return errorResponse(throttleTimeMs, Errors.forException(e));

Review Comment:
   This is an existing issue, but `getErrorResponse` can just be 
`errorResponse`.



##########
core/src/test/java/kafka/metrics/ClientMetricsInstanceTest.java:
##########
@@ -0,0 +1,104 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package kafka.metrics;
+
+import org.apache.kafka.common.Uuid;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import java.net.UnknownHostException;
+
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+public class ClientMetricsInstanceTest {
+
+    private Uuid uuid;
+    private ClientMetricsInstanceMetadata instanceMetadata;
+    private ClientMetricsInstance clientInstance;
+
+    @BeforeEach
+    public void setUp() throws UnknownHostException {
+        uuid = Uuid.randomUuid();
+        instanceMetadata = new ClientMetricsInstanceMetadata(uuid,
+            ClientMetricsTestUtils.requestContext());

Review Comment:
   merge with previous line?



##########
core/src/test/java/kafka/server/ClientMetricsManagerTest.java:
##########
@@ -0,0 +1,921 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package kafka.server;
+
+import kafka.metrics.ClientMetricsConfigs;
+import kafka.metrics.ClientMetricsInstance;
+import kafka.metrics.ClientMetricsTestUtils;
+import kafka.server.ClientMetricsManager.SubscriptionInfo;
+
+import org.apache.kafka.common.Uuid;
+import org.apache.kafka.common.message.GetTelemetrySubscriptionsRequestData;
+import org.apache.kafka.common.message.PushTelemetryRequestData;
+import org.apache.kafka.common.protocol.Errors;
+import org.apache.kafka.common.record.CompressionType;
+import org.apache.kafka.common.requests.GetTelemetrySubscriptionsRequest;
+import org.apache.kafka.common.requests.GetTelemetrySubscriptionsResponse;
+import org.apache.kafka.common.requests.PushTelemetryRequest;
+import org.apache.kafka.common.requests.PushTelemetryRequest.Builder;
+import org.apache.kafka.common.requests.PushTelemetryResponse;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import java.net.UnknownHostException;
+import java.nio.charset.StandardCharsets;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.List;
+import java.util.Properties;
+import java.util.Set;
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.TimeUnit;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+public class ClientMetricsManagerTest {
+
+    private ClientMetricsManager clientMetricsManager;
+
+    @BeforeEach
+    public void setUp() {
+        clientMetricsManager = new ClientMetricsManager();
+    }
+
+    @Test
+    public void testUpdateSubscription() {
+        assertTrue(clientMetricsManager.subscriptions().isEmpty());
+
+        long previousEpoch = 
clientMetricsManager.lastSubscriptionUpdateEpoch();
+        clientMetricsManager.updateSubscription("sub-1", 
ClientMetricsTestUtils.defaultProperties());
+
+        assertEquals(1, clientMetricsManager.subscriptions().size());
+        assertNotNull(clientMetricsManager.subscriptionInfo("sub-1"));
+
+        SubscriptionInfo subscriptionInfo = 
clientMetricsManager.subscriptionInfo("sub-1");
+        Set<String> metrics = subscriptionInfo.metrics();
+
+        // Validate metrics.
+        assertEquals(ClientMetricsTestUtils.DEFAULT_METRICS.split(",").length, 
metrics.size());
+        
Arrays.stream(ClientMetricsTestUtils.DEFAULT_METRICS.split(",")).forEach(metric 
->
+            assertTrue(metrics.contains(metric)));
+        // Validate push interval.
+        
assertEquals(ClientMetricsTestUtils.defaultProperties().getProperty(ClientMetricsConfigs.PUSH_INTERVAL_MS),
+            String.valueOf(subscriptionInfo.intervalMs()));
+
+        // Validate match patterns.
+        
assertEquals(ClientMetricsTestUtils.DEFAULT_CLIENT_MATCH_PATTERNS.size(),
+            subscriptionInfo.matchPattern().size());
+        ClientMetricsTestUtils.DEFAULT_CLIENT_MATCH_PATTERNS.forEach(pattern 
-> {
+            String[] split = pattern.split("=");
+            assertTrue(subscriptionInfo.matchPattern().containsKey(split[0]));
+            assertEquals(split[1], 
subscriptionInfo.matchPattern().get(split[0]).pattern());
+        });
+        assertTrue(clientMetricsManager.lastSubscriptionUpdateEpoch() > 
previousEpoch);
+    }
+
+    @Test
+    public void testUpdateSubscriptionWithEmptyProperties() {
+        assertTrue(clientMetricsManager.subscriptions().isEmpty());
+        long previousEpoch = 
clientMetricsManager.lastSubscriptionUpdateEpoch();
+        clientMetricsManager.updateSubscription("sub-1", new Properties());
+        // No subscription should be added as the properties are empty.
+        assertEquals(0, clientMetricsManager.subscriptions().size());
+        assertEquals(previousEpoch, 
clientMetricsManager.lastSubscriptionUpdateEpoch());

Review Comment:
   `clientMetricsManager.lastSubscriptionUpdateEpoch()` could  change during 
the test, right? Could we use MockTime to make the test more reliable?



##########
core/src/main/java/kafka/server/ClientMetricsManager.java:
##########
@@ -30,17 +68,366 @@ public class ClientMetricsManager implements Closeable {
 
     private static final Logger log = 
LoggerFactory.getLogger(ClientMetricsManager.class);
     private static final ClientMetricsManager INSTANCE = new 
ClientMetricsManager();
+    private static final List<Byte> SUPPORTED_COMPRESSION_TYPES = 
Collections.unmodifiableList(
+        Arrays.asList(CompressionType.ZSTD.id, CompressionType.LZ4.id, 
CompressionType.GZIP.id, CompressionType.SNAPPY.id));
 
     public static ClientMetricsManager instance() {
         return INSTANCE;
     }
+    // Max cache size (16k active client connections per broker)
+    private static final int CM_CACHE_MAX_SIZE = 16384;
+    private final Cache<Uuid, ClientMetricsInstance> clientInstanceCache;
+    private final Map<String, SubscriptionInfo> subscriptionMap;
+
+    // The last subscription updated time is used to determine if the next 
telemetry request needs
+    // to re-evaluate the subscription id as per changes subscriptions.
+    private volatile long lastSubscriptionUpdateEpoch;
+
+    // Visible for testing
+    ClientMetricsManager() {
+        subscriptionMap = new ConcurrentHashMap<>();
+        clientInstanceCache = new SynchronizedCache<>(new 
LRUCache<>(CM_CACHE_MAX_SIZE));
+    }
 
     public void updateSubscription(String subscriptionName, Properties 
properties) {
-        // TODO: Implement the update logic to manage subscriptions.
+        // IncrementalAlterConfigs API will send empty configs when all the 
configs are deleted
+        // for respective subscription. In that case, we need to remove the 
subscription from the map.
+        if (properties.isEmpty()) {
+            // Remove the subscription from the map if it exists, else ignore 
the config update.
+            if (subscriptionMap.containsKey(subscriptionName)) {
+                log.info("Removing subscription [{}] from the subscription 
map", subscriptionName);
+                subscriptionMap.remove(subscriptionName);
+                updateLastSubscriptionUpdateEpoch();
+            }
+            return;
+        }
+
+        ClientMetricsConfigs configs = new ClientMetricsConfigs(properties);
+        updateClientSubscription(subscriptionName, configs);
+        /*
+         Update last subscription updated time to current time to indicate 
that there is a change
+         in the subscription. This will be used to determine if the next 
telemetry request needs
+         to re-evaluate the subscription id as per changes subscriptions.
+        */
+        updateLastSubscriptionUpdateEpoch();
+    }
+
+    public GetTelemetrySubscriptionsResponse 
processGetTelemetrySubscriptionRequest(
+        GetTelemetrySubscriptionsRequest request, int telemetryMaxBytes, 
RequestContext requestContext, int throttleMs) {
+
+        long now = System.currentTimeMillis();
+        Uuid clientInstanceId = 
Optional.ofNullable(request.data().clientInstanceId())
+            .filter(id -> !id.equals(Uuid.ZERO_UUID))
+            .orElse(generateNewClientId());
+
+        /*
+         Get the client instance from the cache or create a new one. If 
subscription has changed
+         since the last request, then the client instance will be 
re-evaluated. Validation of the
+         request will be done after the client instance is created. If client 
issued get telemetry
+         request prior to push interval, then the client should get a throttle 
error but if the
+         subscription has changed since the last request then the client 
should get the updated
+         subscription immediately.
+        */
+        ClientMetricsInstance clientInstance = 
clientInstance(clientInstanceId, requestContext, now);
+
+        try {
+            // Validate the get request parameters for the client instance.
+            validateGetRequest(request, clientInstance, now);
+        } catch (ApiException exception) {
+            return request.getErrorResponse(throttleMs, exception);
+        }
+
+        clientInstance.lastKnownError(Errors.NONE);
+        return createGetSubscriptionResponse(clientInstanceId, clientInstance, 
telemetryMaxBytes, throttleMs);
+    }
+
+    public PushTelemetryResponse 
processPushTelemetryRequest(PushTelemetryRequest request,
+        int telemetryMaxBytes, RequestContext requestContext, int throttleMs) {
+
+        Uuid clientInstanceId = request.data().clientInstanceId();
+        if (clientInstanceId == null || clientInstanceId == Uuid.ZERO_UUID) {
+            String msg = String.format("Invalid request from the client [%s], 
invalid client instance id",
+                clientInstanceId);
+            return request.getErrorResponse(throttleMs, new 
InvalidRequestException(msg));
+        }
+
+        long now = System.currentTimeMillis();
+        ClientMetricsInstance clientInstance = 
clientInstance(clientInstanceId, requestContext, now);
+
+        try {
+            // Validate the push request parameters for the client instance.
+            validatePushRequest(request, telemetryMaxBytes, clientInstance, 
now);
+        } catch (ApiException exception) {
+            log.debug("Error validating push telemetry request from client 
[{}]", clientInstanceId, exception);
+            clientInstance.lastKnownError(Errors.forException(exception));
+            return request.getErrorResponse(throttleMs, exception);
+        } finally {
+            // Update the client instance with the latest push request 
parameters.
+            clientInstance.terminating(request.data().terminating());
+        }
+
+        // Push the metrics to the external client receiver plugin.
+        byte[] metrics = request.data().metrics();
+        if (metrics != null && metrics.length > 0) {
+            try {
+                
ClientMetricsReceiverPlugin.instance().exportMetrics(requestContext, request);
+            } catch (Exception exception) {
+                clientInstance.lastKnownError(Errors.INVALID_RECORD);
+                return request.errorResponse(throttleMs, 
Errors.INVALID_RECORD);
+            }
+        }
+
+        clientInstance.lastKnownError(Errors.NONE);
+        return new PushTelemetryResponse(new 
PushTelemetryResponseData().setThrottleTimeMs(throttleMs));
     }
 
     @Override
     public void close() throws IOException {
-        // TODO: Implement the close logic to close the client metrics manager.
+        subscriptionMap.clear();
+    }
+
+    private synchronized void updateLastSubscriptionUpdateEpoch() {
+        this.lastSubscriptionUpdateEpoch = System.currentTimeMillis();
+    }
+
+    private void updateClientSubscription(String subscriptionName, 
ClientMetricsConfigs configs) {
+        List<String> metrics = 
configs.getList(ClientMetricsConfigs.SUBSCRIPTION_METRICS);
+        int pushInterval = 
configs.getInt(ClientMetricsConfigs.PUSH_INTERVAL_MS);
+        List<String> clientMatchPattern = 
configs.getList(ClientMetricsConfigs.CLIENT_MATCH_PATTERN);
+
+        SubscriptionInfo newSubscription =
+            new SubscriptionInfo(subscriptionName, metrics, pushInterval,
+                
ClientMetricsConfigs.parseMatchingPatterns(clientMatchPattern));
+
+        subscriptionMap.put(subscriptionName, newSubscription);
+    }
+
+    private Uuid generateNewClientId() {
+        Uuid id = Uuid.randomUuid();
+        while (clientInstanceCache.get(id) != null) {
+            id = Uuid.randomUuid();
+        }
+        return id;
+    }
+
+    private ClientMetricsInstance clientInstance(Uuid clientInstanceId, 
RequestContext requestContext,
+        long timestamp) {
+        ClientMetricsInstance clientInstance = 
clientInstanceCache.get(clientInstanceId);
+
+        if (clientInstance == null) {
+            /*
+             If the client instance is not present in the cache, then create a 
new client instance
+             and update the cache. This can also happen when the telemetry 
request is received by
+             the separate broker instance. Though cache is synchronized, but 
it is possible that
+             concurrent calls can create the same client instance. Hence, 
safeguard the client instance
+             creation with a double-checked lock to ensure that only one 
instance is created.
+            */
+            synchronized (this) {
+                clientInstance = clientInstanceCache.get(clientInstanceId);
+                if (clientInstance != null) {
+                    return clientInstance;
+                }
+
+                ClientMetricsInstanceMetadata instanceMetadata = new 
ClientMetricsInstanceMetadata(
+                    clientInstanceId, requestContext);
+                clientInstance = 
createClientInstanceAndUpdateCache(clientInstanceId, instanceMetadata, 
timestamp);
+            }
+        } else if (clientInstance.subscriptionUpdateEpoch() < 
lastSubscriptionUpdateEpoch) {
+            /*
+             If the last subscription update time for client instance is older 
than the subscription
+             updated time, then re-evaluate the subscription information for 
the client as per the
+             updated subscriptions. This is to ensure that the client instance 
is always in sync with
+             the latest subscription information. Though cache is 
synchronized, but it is possible that
+             concurrent calls can create the same client instance. Hence, 
safeguard the client instance
+             update with a double-checked lock to ensure that only one 
instance is created.
+            */
+            synchronized (this) {
+                clientInstance = clientInstanceCache.get(clientInstanceId);
+                if (clientInstance.subscriptionUpdateEpoch() >= 
lastSubscriptionUpdateEpoch) {
+                    return clientInstance;
+                }
+                clientInstance = 
createClientInstanceAndUpdateCache(clientInstanceId, 
clientInstance.instanceMetadata(), timestamp);
+            }
+        }
+
+        return clientInstance;
+    }
+
+    private ClientMetricsInstance createClientInstanceAndUpdateCache(Uuid 
clientInstanceId,
+        ClientMetricsInstanceMetadata instanceMetadata, long timestamp) {
+
+        ClientMetricsInstance clientInstance = 
createClientInstance(clientInstanceId, instanceMetadata,
+            timestamp);
+        clientInstanceCache.put(clientInstanceId, clientInstance);
+        return clientInstance;
+    }
+
+    private ClientMetricsInstance createClientInstance(Uuid clientInstanceId,
+        ClientMetricsInstanceMetadata instanceMetadata, long timestamp) {
+
+        int pushIntervalMs = ClientMetricsConfigs.DEFAULT_INTERVAL_MS;
+        // Keep a set of metrics to avoid duplicates in case of overlapping 
subscriptions.
+        Set<String> subscribedMetrics = new HashSet<>();
+        boolean allMetricsSubscribed = false;
+
+        for (SubscriptionInfo info : subscriptions()) {
+            if (instanceMetadata.isMatch(info.matchPattern())) {
+                allMetricsSubscribed = allMetricsSubscribed || 
info.metrics().contains(
+                    ClientMetricsConfigs.ALL_SUBSCRIBED_METRICS_CONFIG);
+                subscribedMetrics.addAll(info.metrics());
+                pushIntervalMs = Math.min(pushIntervalMs, info.intervalMs());
+            }
+        }
+
+        /*
+         If client matches with any subscription that has empty metrics 
string, then it means that client
+         is subscribed to all the metrics, so just send the empty string as 
the subscribed metrics.
+        */
+        if (allMetricsSubscribed) {
+            subscribedMetrics.clear();
+            // Add an empty string to indicate that all metrics are subscribed.
+            subscribedMetrics.add(ClientMetricsConfigs.ALL_SUBSCRIBED_METRICS);
+        }
+
+        int subscriptionId = computeSubscriptionId(subscribedMetrics, 
pushIntervalMs, clientInstanceId);
+
+        return new ClientMetricsInstance(clientInstanceId, instanceMetadata, 
subscriptionId, timestamp,
+            subscribedMetrics, pushIntervalMs);
+    }
+
+    /**
+     * Computes the SubscriptionId as a unique identifier for a client 
instance's subscription set,
+     * the id is generated by calculating a CRC32 of the configured metrics 
subscriptions including
+     * the PushIntervalMs, XORed with the ClientInstanceId.
+     */
+    private int computeSubscriptionId(Set<String> metrics, int pushIntervalMs, 
Uuid clientInstanceId) {
+        CRC32 crc = new CRC32();

Review Comment:
   We have moved to CRC32C since it's more efficient 
(https://issues.apache.org/jira/browse/KAFKA-1449). Should we use CRC32C here 
too?



##########
core/src/test/java/kafka/metrics/ClientMetricsInstanceTest.java:
##########
@@ -0,0 +1,104 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package kafka.metrics;
+
+import org.apache.kafka.common.Uuid;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import java.net.UnknownHostException;
+
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+public class ClientMetricsInstanceTest {
+
+    private Uuid uuid;
+    private ClientMetricsInstanceMetadata instanceMetadata;
+    private ClientMetricsInstance clientInstance;
+
+    @BeforeEach
+    public void setUp() throws UnknownHostException {
+        uuid = Uuid.randomUuid();
+        instanceMetadata = new ClientMetricsInstanceMetadata(uuid,
+            ClientMetricsTestUtils.requestContext());
+        clientInstance = new ClientMetricsInstance(Uuid.randomUuid(), 
instanceMetadata, 0, 0,
+            null, ClientMetricsConfigs.DEFAULT_INTERVAL_MS);
+    }
+
+    @Test
+    public void testMaybeUpdateRequestEpochValid() {
+        // First request should be accepted.
+        
assertTrue(clientInstance.maybeUpdateGetRequestEpoch(System.currentTimeMillis()));
+        
assertTrue(clientInstance.maybeUpdatePushRequestEpoch(System.currentTimeMillis()));
+    }
+
+    @Test
+    public void testMaybeUpdateGetRequestAfterElapsedTimeValid() throws 
InterruptedException {
+        ClientMetricsInstance clientInstance = new ClientMetricsInstance(uuid, 
instanceMetadata, 0, 0,
+            null, 2);
+        
assertTrue(clientInstance.maybeUpdateGetRequestEpoch(System.currentTimeMillis()));
+        // sleep for 3 ms to ensure that the next request is accepted.
+        Thread.sleep(3);
+        // Second request should be accepted as time since last request is 
greater than the retry interval.

Review Comment:
   retry interval => push interval



##########
core/src/test/java/kafka/metrics/ClientMetricsInstanceMetadataTest.java:
##########
@@ -0,0 +1,136 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package kafka.metrics;
+
+import org.apache.kafka.common.Uuid;
+import org.junit.jupiter.api.Test;
+
+import java.net.InetAddress;
+import java.net.UnknownHostException;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.Map;
+import java.util.regex.Pattern;
+
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+public class ClientMetricsInstanceMetadataTest {
+
+    @Test
+    public void testIsMatchValid() throws UnknownHostException {
+        Uuid uuid = Uuid.randomUuid();
+        ClientMetricsInstanceMetadata instanceMetadata = new 
ClientMetricsInstanceMetadata(uuid,
+            ClientMetricsTestUtils.requestContext());
+        // We consider empty/missing client matching patterns as valid
+        assertTrue(instanceMetadata.isMatch(Collections.emptyMap()));
+        assertTrue(instanceMetadata.isMatch(null));

Review Comment:
   How could a user create a null pattern?



##########
core/src/test/java/kafka/metrics/ClientMetricsInstanceTest.java:
##########
@@ -0,0 +1,104 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package kafka.metrics;
+
+import org.apache.kafka.common.Uuid;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import java.net.UnknownHostException;
+
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+public class ClientMetricsInstanceTest {
+
+    private Uuid uuid;
+    private ClientMetricsInstanceMetadata instanceMetadata;
+    private ClientMetricsInstance clientInstance;
+
+    @BeforeEach
+    public void setUp() throws UnknownHostException {
+        uuid = Uuid.randomUuid();
+        instanceMetadata = new ClientMetricsInstanceMetadata(uuid,
+            ClientMetricsTestUtils.requestContext());
+        clientInstance = new ClientMetricsInstance(Uuid.randomUuid(), 
instanceMetadata, 0, 0,
+            null, ClientMetricsConfigs.DEFAULT_INTERVAL_MS);
+    }
+
+    @Test
+    public void testMaybeUpdateRequestEpochValid() {
+        // First request should be accepted.
+        
assertTrue(clientInstance.maybeUpdateGetRequestEpoch(System.currentTimeMillis()));
+        
assertTrue(clientInstance.maybeUpdatePushRequestEpoch(System.currentTimeMillis()));
+    }
+
+    @Test
+    public void testMaybeUpdateGetRequestAfterElapsedTimeValid() throws 
InterruptedException {
+        ClientMetricsInstance clientInstance = new ClientMetricsInstance(uuid, 
instanceMetadata, 0, 0,
+            null, 2);
+        
assertTrue(clientInstance.maybeUpdateGetRequestEpoch(System.currentTimeMillis()));
+        // sleep for 3 ms to ensure that the next request is accepted.
+        Thread.sleep(3);
+        // Second request should be accepted as time since last request is 
greater than the retry interval.
+        
assertTrue(clientInstance.maybeUpdateGetRequestEpoch(System.currentTimeMillis()));
+    }
+
+    @Test
+    public void testMaybeUpdateGetRequestWithImmediateRetryFail() {
+        
assertTrue(clientInstance.maybeUpdateGetRequestEpoch(System.currentTimeMillis()));
+        // Second request should be rejected as time since last request is 
less than the retry interval.
+        
assertFalse(clientInstance.maybeUpdateGetRequestEpoch(System.currentTimeMillis()));
+    }
+
+    @Test
+    public void testMaybeUpdatePushRequestAfterElapsedTimeValid() throws 
InterruptedException {

Review Comment:
   How is this test different from 
`testMaybeUpdateGetRequestAfterElapsedTimeValid`?



##########
core/src/test/java/kafka/metrics/ClientMetricsReceiverPluginTest.java:
##########
@@ -0,0 +1,56 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package kafka.metrics;
+
+import kafka.metrics.ClientMetricsTestUtils.TestClientMetricsReceiver;
+import org.apache.kafka.common.message.PushTelemetryRequestData;
+import org.apache.kafka.common.requests.PushTelemetryRequest;
+import org.junit.jupiter.api.Test;
+
+import java.net.UnknownHostException;
+import java.nio.charset.StandardCharsets;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+public class ClientMetricsReceiverPluginTest {
+
+    TestClientMetricsReceiver telemetryReceiver = new 
TestClientMetricsReceiver();
+
+    @Test
+    public void testExportMetrics() throws UnknownHostException {
+        assertTrue(ClientMetricsReceiverPlugin.instance().isEmpty());
+
+        ClientMetricsReceiverPlugin.instance().add(telemetryReceiver);
+        assertFalse(ClientMetricsReceiverPlugin.instance().isEmpty());
+
+        assertEquals(0, telemetryReceiver.exportMetricsInvokedCount);
+        assertTrue(telemetryReceiver.metricsData.isEmpty());
+
+        
ClientMetricsReceiverPlugin.instance().exportMetrics(ClientMetricsTestUtils.requestContext(),
+            new PushTelemetryRequest.Builder(
+                new PushTelemetryRequestData()
+                    
.setMetrics("test-metrics".getBytes(StandardCharsets.UTF_8)))
+                .build());
+
+        assertEquals(1, telemetryReceiver.exportMetricsInvokedCount);
+        assertEquals(1, telemetryReceiver.metricsData.size());
+        assertEquals("test-metrics", new String(

Review Comment:
   Could we just directly compare the bytes?



##########
core/src/main/java/kafka/metrics/ClientMetricsReceiverPlugin.java:
##########
@@ -0,0 +1,62 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package kafka.metrics;
+
+import org.apache.kafka.common.requests.PushTelemetryRequest;
+import org.apache.kafka.common.requests.RequestContext;
+import org.apache.kafka.server.telemetry.ClientTelemetryReceiver;
+
+import java.util.ArrayList;
+import java.util.List;
+
+/**
+ * Plugin to register client telemetry receivers and export metrics. This 
class is used by the Kafka
+ * server to export client metrics to the registered receivers.
+ */
+public class ClientMetricsReceiverPlugin {
+
+    private static final ClientMetricsReceiverPlugin INSTANCE = new 
ClientMetricsReceiverPlugin();

Review Comment:
   static fields like that could make it more difficult to test multiple broker 
instances in the same JVM. Could we create an instance when creating a broker 
and pass that instance along? Ditto for `ClientMetricsManager.INSTANCE`.



##########
core/src/test/java/kafka/metrics/ClientMetricsInstanceTest.java:
##########
@@ -0,0 +1,104 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package kafka.metrics;
+
+import org.apache.kafka.common.Uuid;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import java.net.UnknownHostException;
+
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+public class ClientMetricsInstanceTest {
+
+    private Uuid uuid;
+    private ClientMetricsInstanceMetadata instanceMetadata;
+    private ClientMetricsInstance clientInstance;
+
+    @BeforeEach
+    public void setUp() throws UnknownHostException {
+        uuid = Uuid.randomUuid();
+        instanceMetadata = new ClientMetricsInstanceMetadata(uuid,
+            ClientMetricsTestUtils.requestContext());
+        clientInstance = new ClientMetricsInstance(Uuid.randomUuid(), 
instanceMetadata, 0, 0,
+            null, ClientMetricsConfigs.DEFAULT_INTERVAL_MS);
+    }
+
+    @Test
+    public void testMaybeUpdateRequestEpochValid() {
+        // First request should be accepted.
+        
assertTrue(clientInstance.maybeUpdateGetRequestEpoch(System.currentTimeMillis()));
+        
assertTrue(clientInstance.maybeUpdatePushRequestEpoch(System.currentTimeMillis()));
+    }
+
+    @Test
+    public void testMaybeUpdateGetRequestAfterElapsedTimeValid() throws 
InterruptedException {
+        ClientMetricsInstance clientInstance = new ClientMetricsInstance(uuid, 
instanceMetadata, 0, 0,
+            null, 2);
+        
assertTrue(clientInstance.maybeUpdateGetRequestEpoch(System.currentTimeMillis()));
+        // sleep for 3 ms to ensure that the next request is accepted.
+        Thread.sleep(3);

Review Comment:
   Could we use MockTime to avoid actual sleeping during tests?



##########
core/src/test/java/kafka/server/ClientMetricsManagerTest.java:
##########
@@ -0,0 +1,921 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package kafka.server;
+
+import kafka.metrics.ClientMetricsConfigs;
+import kafka.metrics.ClientMetricsInstance;
+import kafka.metrics.ClientMetricsTestUtils;
+import kafka.server.ClientMetricsManager.SubscriptionInfo;
+
+import org.apache.kafka.common.Uuid;
+import org.apache.kafka.common.message.GetTelemetrySubscriptionsRequestData;
+import org.apache.kafka.common.message.PushTelemetryRequestData;
+import org.apache.kafka.common.protocol.Errors;
+import org.apache.kafka.common.record.CompressionType;
+import org.apache.kafka.common.requests.GetTelemetrySubscriptionsRequest;
+import org.apache.kafka.common.requests.GetTelemetrySubscriptionsResponse;
+import org.apache.kafka.common.requests.PushTelemetryRequest;
+import org.apache.kafka.common.requests.PushTelemetryRequest.Builder;
+import org.apache.kafka.common.requests.PushTelemetryResponse;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import java.net.UnknownHostException;
+import java.nio.charset.StandardCharsets;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.List;
+import java.util.Properties;
+import java.util.Set;
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.TimeUnit;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+public class ClientMetricsManagerTest {
+
+    private ClientMetricsManager clientMetricsManager;
+
+    @BeforeEach
+    public void setUp() {
+        clientMetricsManager = new ClientMetricsManager();
+    }
+
+    @Test
+    public void testUpdateSubscription() {
+        assertTrue(clientMetricsManager.subscriptions().isEmpty());
+
+        long previousEpoch = 
clientMetricsManager.lastSubscriptionUpdateEpoch();
+        clientMetricsManager.updateSubscription("sub-1", 
ClientMetricsTestUtils.defaultProperties());
+
+        assertEquals(1, clientMetricsManager.subscriptions().size());
+        assertNotNull(clientMetricsManager.subscriptionInfo("sub-1"));
+
+        SubscriptionInfo subscriptionInfo = 
clientMetricsManager.subscriptionInfo("sub-1");
+        Set<String> metrics = subscriptionInfo.metrics();
+
+        // Validate metrics.
+        assertEquals(ClientMetricsTestUtils.DEFAULT_METRICS.split(",").length, 
metrics.size());
+        
Arrays.stream(ClientMetricsTestUtils.DEFAULT_METRICS.split(",")).forEach(metric 
->
+            assertTrue(metrics.contains(metric)));
+        // Validate push interval.
+        
assertEquals(ClientMetricsTestUtils.defaultProperties().getProperty(ClientMetricsConfigs.PUSH_INTERVAL_MS),
+            String.valueOf(subscriptionInfo.intervalMs()));
+
+        // Validate match patterns.
+        
assertEquals(ClientMetricsTestUtils.DEFAULT_CLIENT_MATCH_PATTERNS.size(),
+            subscriptionInfo.matchPattern().size());
+        ClientMetricsTestUtils.DEFAULT_CLIENT_MATCH_PATTERNS.forEach(pattern 
-> {
+            String[] split = pattern.split("=");
+            assertTrue(subscriptionInfo.matchPattern().containsKey(split[0]));
+            assertEquals(split[1], 
subscriptionInfo.matchPattern().get(split[0]).pattern());
+        });
+        assertTrue(clientMetricsManager.lastSubscriptionUpdateEpoch() > 
previousEpoch);
+    }
+
+    @Test
+    public void testUpdateSubscriptionWithEmptyProperties() {
+        assertTrue(clientMetricsManager.subscriptions().isEmpty());
+        long previousEpoch = 
clientMetricsManager.lastSubscriptionUpdateEpoch();
+        clientMetricsManager.updateSubscription("sub-1", new Properties());
+        // No subscription should be added as the properties are empty.
+        assertEquals(0, clientMetricsManager.subscriptions().size());
+        assertEquals(previousEpoch, 
clientMetricsManager.lastSubscriptionUpdateEpoch());
+    }
+
+    @Test
+    public void testUpdateSubscriptionWithNullProperties() {
+        assertTrue(clientMetricsManager.subscriptions().isEmpty());
+        long previousEpoch = 
clientMetricsManager.lastSubscriptionUpdateEpoch();
+        // Properties shouldn't be passed as null.
+        assertThrows(NullPointerException.class, () ->
+            clientMetricsManager.updateSubscription("sub-1", null));
+        assertEquals(0, clientMetricsManager.subscriptions().size());
+        assertEquals(previousEpoch, 
clientMetricsManager.lastSubscriptionUpdateEpoch());
+    }
+
+    @Test
+    public void testUpdateSubscriptionWithInvalidMetricsProperties() {
+        assertTrue(clientMetricsManager.subscriptions().isEmpty());
+
+        long previousEpoch = 
clientMetricsManager.lastSubscriptionUpdateEpoch();
+        Properties properties = new Properties();
+        properties.put("random", "random");
+        clientMetricsManager.updateSubscription("sub-1", properties);
+
+        // Undefined keys are ignored.

Review Comment:
   Hmm, this is kind of inconsistent. If we remove all properties in a config, 
we end up removing the subscription. However, if we create a subscription with 
a non-existing property, we end up with a subscription with the default 
properties.



##########
core/src/main/java/kafka/server/ClientMetricsManager.java:
##########
@@ -30,17 +68,366 @@ public class ClientMetricsManager implements Closeable {
 
     private static final Logger log = 
LoggerFactory.getLogger(ClientMetricsManager.class);
     private static final ClientMetricsManager INSTANCE = new 
ClientMetricsManager();
+    private static final List<Byte> SUPPORTED_COMPRESSION_TYPES = 
Collections.unmodifiableList(
+        Arrays.asList(CompressionType.ZSTD.id, CompressionType.LZ4.id, 
CompressionType.GZIP.id, CompressionType.SNAPPY.id));
 
     public static ClientMetricsManager instance() {
         return INSTANCE;
     }
+    // Max cache size (16k active client connections per broker)
+    private static final int CM_CACHE_MAX_SIZE = 16384;
+    private final Cache<Uuid, ClientMetricsInstance> clientInstanceCache;
+    private final Map<String, SubscriptionInfo> subscriptionMap;
+
+    // The last subscription updated time is used to determine if the next 
telemetry request needs
+    // to re-evaluate the subscription id as per changes subscriptions.
+    private volatile long lastSubscriptionUpdateEpoch;
+
+    // Visible for testing
+    ClientMetricsManager() {
+        subscriptionMap = new ConcurrentHashMap<>();
+        clientInstanceCache = new SynchronizedCache<>(new 
LRUCache<>(CM_CACHE_MAX_SIZE));
+    }
 
     public void updateSubscription(String subscriptionName, Properties 
properties) {
-        // TODO: Implement the update logic to manage subscriptions.
+        // IncrementalAlterConfigs API will send empty configs when all the 
configs are deleted
+        // for respective subscription. In that case, we need to remove the 
subscription from the map.
+        if (properties.isEmpty()) {
+            // Remove the subscription from the map if it exists, else ignore 
the config update.
+            if (subscriptionMap.containsKey(subscriptionName)) {
+                log.info("Removing subscription [{}] from the subscription 
map", subscriptionName);
+                subscriptionMap.remove(subscriptionName);
+                updateLastSubscriptionUpdateEpoch();
+            }
+            return;
+        }
+
+        ClientMetricsConfigs configs = new ClientMetricsConfigs(properties);
+        updateClientSubscription(subscriptionName, configs);
+        /*
+         Update last subscription updated time to current time to indicate 
that there is a change
+         in the subscription. This will be used to determine if the next 
telemetry request needs
+         to re-evaluate the subscription id as per changes subscriptions.
+        */
+        updateLastSubscriptionUpdateEpoch();
+    }
+
+    public GetTelemetrySubscriptionsResponse 
processGetTelemetrySubscriptionRequest(
+        GetTelemetrySubscriptionsRequest request, int telemetryMaxBytes, 
RequestContext requestContext, int throttleMs) {
+
+        long now = System.currentTimeMillis();
+        Uuid clientInstanceId = 
Optional.ofNullable(request.data().clientInstanceId())
+            .filter(id -> !id.equals(Uuid.ZERO_UUID))
+            .orElse(generateNewClientId());
+
+        /*
+         Get the client instance from the cache or create a new one. If 
subscription has changed
+         since the last request, then the client instance will be 
re-evaluated. Validation of the
+         request will be done after the client instance is created. If client 
issued get telemetry
+         request prior to push interval, then the client should get a throttle 
error but if the
+         subscription has changed since the last request then the client 
should get the updated
+         subscription immediately.
+        */
+        ClientMetricsInstance clientInstance = 
clientInstance(clientInstanceId, requestContext, now);
+
+        try {
+            // Validate the get request parameters for the client instance.
+            validateGetRequest(request, clientInstance, now);
+        } catch (ApiException exception) {
+            return request.getErrorResponse(throttleMs, exception);
+        }
+
+        clientInstance.lastKnownError(Errors.NONE);
+        return createGetSubscriptionResponse(clientInstanceId, clientInstance, 
telemetryMaxBytes, throttleMs);
+    }
+
+    public PushTelemetryResponse 
processPushTelemetryRequest(PushTelemetryRequest request,
+        int telemetryMaxBytes, RequestContext requestContext, int throttleMs) {
+
+        Uuid clientInstanceId = request.data().clientInstanceId();
+        if (clientInstanceId == null || clientInstanceId == Uuid.ZERO_UUID) {

Review Comment:
   It seems that `clientInstanceId` can't be any in `Uuid.RESERVED`?



##########
core/src/test/java/kafka/server/ClientMetricsManagerTest.java:
##########
@@ -0,0 +1,921 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package kafka.server;
+
+import kafka.metrics.ClientMetricsConfigs;
+import kafka.metrics.ClientMetricsInstance;
+import kafka.metrics.ClientMetricsTestUtils;
+import kafka.server.ClientMetricsManager.SubscriptionInfo;
+
+import org.apache.kafka.common.Uuid;
+import org.apache.kafka.common.message.GetTelemetrySubscriptionsRequestData;
+import org.apache.kafka.common.message.PushTelemetryRequestData;
+import org.apache.kafka.common.protocol.Errors;
+import org.apache.kafka.common.record.CompressionType;
+import org.apache.kafka.common.requests.GetTelemetrySubscriptionsRequest;
+import org.apache.kafka.common.requests.GetTelemetrySubscriptionsResponse;
+import org.apache.kafka.common.requests.PushTelemetryRequest;
+import org.apache.kafka.common.requests.PushTelemetryRequest.Builder;
+import org.apache.kafka.common.requests.PushTelemetryResponse;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import java.net.UnknownHostException;
+import java.nio.charset.StandardCharsets;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.List;
+import java.util.Properties;
+import java.util.Set;
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.TimeUnit;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+public class ClientMetricsManagerTest {
+
+    private ClientMetricsManager clientMetricsManager;
+
+    @BeforeEach
+    public void setUp() {
+        clientMetricsManager = new ClientMetricsManager();
+    }
+
+    @Test
+    public void testUpdateSubscription() {
+        assertTrue(clientMetricsManager.subscriptions().isEmpty());
+
+        long previousEpoch = 
clientMetricsManager.lastSubscriptionUpdateEpoch();
+        clientMetricsManager.updateSubscription("sub-1", 
ClientMetricsTestUtils.defaultProperties());
+
+        assertEquals(1, clientMetricsManager.subscriptions().size());
+        assertNotNull(clientMetricsManager.subscriptionInfo("sub-1"));
+
+        SubscriptionInfo subscriptionInfo = 
clientMetricsManager.subscriptionInfo("sub-1");
+        Set<String> metrics = subscriptionInfo.metrics();
+
+        // Validate metrics.
+        assertEquals(ClientMetricsTestUtils.DEFAULT_METRICS.split(",").length, 
metrics.size());
+        
Arrays.stream(ClientMetricsTestUtils.DEFAULT_METRICS.split(",")).forEach(metric 
->
+            assertTrue(metrics.contains(metric)));
+        // Validate push interval.
+        
assertEquals(ClientMetricsTestUtils.defaultProperties().getProperty(ClientMetricsConfigs.PUSH_INTERVAL_MS),
+            String.valueOf(subscriptionInfo.intervalMs()));
+
+        // Validate match patterns.
+        
assertEquals(ClientMetricsTestUtils.DEFAULT_CLIENT_MATCH_PATTERNS.size(),
+            subscriptionInfo.matchPattern().size());
+        ClientMetricsTestUtils.DEFAULT_CLIENT_MATCH_PATTERNS.forEach(pattern 
-> {
+            String[] split = pattern.split("=");
+            assertTrue(subscriptionInfo.matchPattern().containsKey(split[0]));
+            assertEquals(split[1], 
subscriptionInfo.matchPattern().get(split[0]).pattern());
+        });
+        assertTrue(clientMetricsManager.lastSubscriptionUpdateEpoch() > 
previousEpoch);
+    }
+
+    @Test
+    public void testUpdateSubscriptionWithEmptyProperties() {
+        assertTrue(clientMetricsManager.subscriptions().isEmpty());
+        long previousEpoch = 
clientMetricsManager.lastSubscriptionUpdateEpoch();
+        clientMetricsManager.updateSubscription("sub-1", new Properties());
+        // No subscription should be added as the properties are empty.
+        assertEquals(0, clientMetricsManager.subscriptions().size());
+        assertEquals(previousEpoch, 
clientMetricsManager.lastSubscriptionUpdateEpoch());
+    }
+
+    @Test
+    public void testUpdateSubscriptionWithNullProperties() {
+        assertTrue(clientMetricsManager.subscriptions().isEmpty());
+        long previousEpoch = 
clientMetricsManager.lastSubscriptionUpdateEpoch();
+        // Properties shouldn't be passed as null.
+        assertThrows(NullPointerException.class, () ->
+            clientMetricsManager.updateSubscription("sub-1", null));
+        assertEquals(0, clientMetricsManager.subscriptions().size());
+        assertEquals(previousEpoch, 
clientMetricsManager.lastSubscriptionUpdateEpoch());
+    }
+
+    @Test
+    public void testUpdateSubscriptionWithInvalidMetricsProperties() {
+        assertTrue(clientMetricsManager.subscriptions().isEmpty());
+
+        long previousEpoch = 
clientMetricsManager.lastSubscriptionUpdateEpoch();
+        Properties properties = new Properties();
+        properties.put("random", "random");
+        clientMetricsManager.updateSubscription("sub-1", properties);
+
+        // Undefined keys are ignored.
+        assertEquals(1, clientMetricsManager.subscriptions().size());
+        assertNotNull(clientMetricsManager.subscriptionInfo("sub-1"));
+        // Config def default properties.
+        
assertTrue(clientMetricsManager.subscriptionInfo("sub-1").metrics().isEmpty());
+        assertEquals(ClientMetricsConfigs.DEFAULT_INTERVAL_MS, 
clientMetricsManager.subscriptionInfo("sub-1").intervalMs());
+        
assertTrue(clientMetricsManager.subscriptionInfo("sub-1").matchPattern().isEmpty());
+        assertTrue(clientMetricsManager.lastSubscriptionUpdateEpoch() > 
previousEpoch);
+    }
+
+    @Test
+    public void testUpdateSubscriptionWithPropertiesDeletion() throws 
InterruptedException {
+        assertTrue(clientMetricsManager.subscriptions().isEmpty());
+
+        long previousEpoch = 
clientMetricsManager.lastSubscriptionUpdateEpoch();
+        Properties properties = new Properties();
+        properties.put("interval.ms", "100");
+        clientMetricsManager.updateSubscription("sub-1", properties);
+        assertEquals(1, clientMetricsManager.subscriptions().size());
+        assertNotNull(clientMetricsManager.subscriptionInfo("sub-1"));
+        assertTrue(clientMetricsManager.lastSubscriptionUpdateEpoch() > 
previousEpoch);
+
+        previousEpoch = clientMetricsManager.lastSubscriptionUpdateEpoch();
+        Thread.sleep(1);
+        clientMetricsManager.updateSubscription("sub-1", new Properties());
+        // Subscription should be removed as all properties are removed.
+        assertEquals(0, clientMetricsManager.subscriptions().size());
+        assertTrue(clientMetricsManager.lastSubscriptionUpdateEpoch() > 
previousEpoch);
+    }
+
+    @Test
+    public void testGetTelemetry() throws UnknownHostException {
+        clientMetricsManager.updateSubscription("sub-1", 
ClientMetricsTestUtils.defaultProperties());
+        assertEquals(1, clientMetricsManager.subscriptions().size());
+
+        GetTelemetrySubscriptionsRequest request = new 
GetTelemetrySubscriptionsRequest.Builder(
+            new GetTelemetrySubscriptionsRequestData()).build();
+
+        GetTelemetrySubscriptionsResponse response = 
clientMetricsManager.processGetTelemetrySubscriptionRequest(
+            request, 100, ClientMetricsTestUtils.requestContext(), 0);
+
+        assertNotNull(response.data().clientInstanceId());
+        assertTrue(response.data().subscriptionId() != 0);
+
+        assertEquals(ClientMetricsTestUtils.DEFAULT_METRICS.split(",").length, 
response.data().requestedMetrics().size());
+        
Arrays.stream(ClientMetricsTestUtils.DEFAULT_METRICS.split(",")).forEach(metric 
->
+            assertTrue(response.data().requestedMetrics().contains(metric)));
+
+        assertEquals(4, response.data().acceptedCompressionTypes().size());
+        // validate compression types order.
+        assertEquals(CompressionType.ZSTD.id, 
response.data().acceptedCompressionTypes().get(0));
+        assertEquals(CompressionType.LZ4.id, 
response.data().acceptedCompressionTypes().get(1));
+        assertEquals(CompressionType.GZIP.id, 
response.data().acceptedCompressionTypes().get(2));
+        assertEquals(CompressionType.SNAPPY.id, 
response.data().acceptedCompressionTypes().get(3));
+        assertEquals(ClientMetricsTestUtils.DEFAULT_PUSH_INTERVAL_MS, 
response.data().pushIntervalMs());
+        assertTrue(response.data().deltaTemporality());
+        assertEquals(100, response.data().telemetryMaxBytes());
+        assertEquals(Errors.NONE, response.error());
+
+        ClientMetricsInstance instance = 
clientMetricsManager.clientInstance(response.data().clientInstanceId());
+        assertNotNull(instance);
+        assertEquals(Errors.NONE, instance.lastKnownError());
+    }
+
+    @Test
+    public void testGetTelemetryWithoutSubscription() throws 
UnknownHostException {

Review Comment:
   Does this test provide more coverage than `testGetTelemetry()`?



##########
core/src/main/java/kafka/metrics/ClientMetricsInstance.java:
##########
@@ -0,0 +1,122 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package kafka.metrics;
+
+import org.apache.kafka.common.Uuid;
+import org.apache.kafka.common.protocol.Errors;
+
+import java.util.Objects;
+import java.util.Set;
+
+/**
+ * Contains the metrics instance metadata and the state of the client instance.
+ */
+public class ClientMetricsInstance {
+
+    private final Uuid clientInstanceId;
+    private final ClientMetricsInstanceMetadata instanceMetadata;
+    private final int subscriptionId;
+    private final long subscriptionUpdateEpoch;
+    private final Set<String> metrics;
+    private final int pushIntervalMs;
+
+    private long lastGetRequestEpoch;
+    private long lastPushRequestEpoch;
+    private volatile boolean terminating;
+    private volatile Errors lastKnownError;
+
+    public ClientMetricsInstance(Uuid clientInstanceId, 
ClientMetricsInstanceMetadata instanceMetadata,
+        int subscriptionId, long subscriptionUpdateEpoch, Set<String> metrics, 
int pushIntervalMs) {
+        this.clientInstanceId = Objects.requireNonNull(clientInstanceId);
+        this.instanceMetadata = Objects.requireNonNull(instanceMetadata);
+        this.subscriptionId = subscriptionId;
+        this.subscriptionUpdateEpoch = subscriptionUpdateEpoch;
+        this.metrics = metrics;
+        this.terminating = false;
+        this.pushIntervalMs = pushIntervalMs;
+        this.lastKnownError = Errors.NONE;
+    }
+
+    public Uuid clientInstanceId() {
+        return clientInstanceId;
+    }
+
+    public ClientMetricsInstanceMetadata instanceMetadata() {
+        return instanceMetadata;
+    }
+
+    public int pushIntervalMs() {
+        return pushIntervalMs;
+    }
+
+    public long subscriptionUpdateEpoch() {
+        return subscriptionUpdateEpoch;
+    }
+
+    public int subscriptionId() {
+        return subscriptionId;
+    }
+
+    public Set<String> metrics() {
+        return metrics;
+    }
+
+    public boolean terminating() {
+        return terminating;

Review Comment:
   After the instance is terminated, when do we remove the instance form the 
in-memory state?



##########
core/src/main/java/kafka/server/ClientMetricsManager.java:
##########
@@ -34,13 +70,348 @@ public class ClientMetricsManager implements Closeable {
     public static ClientMetricsManager instance() {
         return INSTANCE;
     }
+    // Max cache size (16k active client connections per broker)
+    private static final int CM_CACHE_MAX_SIZE = 16384;
+    private final Cache<Uuid, ClientMetricsInstance> clientInstanceCache;
+    private final Map<String, SubscriptionInfo> subscriptionMap;
+
+    // The last subscription updated time is used to determine if the next 
telemetry request needs
+    // to re-evaluate the subscription id as per changes subscriptions.
+    private long lastSubscriptionUpdateEpoch;
+
+    // Visible for testing
+    ClientMetricsManager() {
+        subscriptionMap = new ConcurrentHashMap<>();
+        clientInstanceCache = new SynchronizedCache<>(new 
LRUCache<>(CM_CACHE_MAX_SIZE));
+    }
 
     public void updateSubscription(String subscriptionName, Properties 
properties) {
-        // TODO: Implement the update logic to manage subscriptions.
+        // IncrementalAlterConfigs API will send empty configs when all the 
configs are deleted
+        // for respective subscription. In that case, we need to remove the 
subscription from the map.
+        if (properties.isEmpty()) {
+            // Remove the subscription from the map if it exists, else ignore 
the config update.
+            if (subscriptionMap.containsKey(subscriptionName)) {
+                log.info("Removing subscription [{}] from the subscription 
map", subscriptionName);
+                subscriptionMap.remove(subscriptionName);
+                updateLastSubscriptionUpdateEpoch();
+            }
+            return;
+        }
+
+        ClientMetricsConfigs configs = new ClientMetricsConfigs(properties);
+        updateClientSubscription(subscriptionName, configs);
+        /*
+         Update last subscription updated time to current time to indicate 
that there is a change
+         in the subscription. This will be used to determine if the next 
telemetry request needs
+         to re-evaluate the subscription id as per changes subscriptions.
+        */
+        updateLastSubscriptionUpdateEpoch();
+    }
+
+    public GetTelemetrySubscriptionsResponse 
processGetTelemetrySubscriptionRequest(
+        GetTelemetrySubscriptionsRequest request, int telemetryMaxBytes, 
RequestContext requestContext, int throttleMs) {
+
+        long now = System.currentTimeMillis();
+        Uuid clientInstanceId = 
Optional.ofNullable(request.data().clientInstanceId())
+            .filter(id -> !id.equals(Uuid.ZERO_UUID))
+            .orElse(generateNewClientId());
+
+        /*
+         Get the client instance from the cache or create a new one. If 
subscription has changed
+         since the last request, then the client instance will be 
re-evaluated. Validation of the
+         request will be done after the client instance is created. If client 
issued get telemetry
+         request prior to push interval, then the client should get a throttle 
error but if the
+         subscription has changed since the last request then the client 
should get the updated
+         subscription immediately.
+        */
+        ClientMetricsInstance clientInstance = 
getClientInstance(clientInstanceId, requestContext, now);
+
+        try {
+            // Validate the get request parameters for the client instance.
+            validateGetRequest(request, clientInstance);
+        } catch (ApiException exception) {
+            return request.getErrorResponse(throttleMs, exception);
+        } finally {
+            clientInstance.lastGetRequestEpoch(now);
+        }
+
+        clientInstance.lastKnownError(Errors.NONE);
+        return createGetSubscriptionResponse(clientInstanceId, clientInstance, 
telemetryMaxBytes, throttleMs);
+    }
+
+    public PushTelemetryResponse 
processPushTelemetryRequest(PushTelemetryRequest request,
+        int telemetryMaxBytes, RequestContext requestContext, int throttleMs) {
+
+        Uuid clientInstanceId = request.data().clientInstanceId();
+        if (clientInstanceId == null || clientInstanceId == Uuid.ZERO_UUID) {
+            String msg = String.format("Invalid request from the client [%s], 
invalid client instance id",
+                clientInstanceId);
+            return request.getErrorResponse(throttleMs, new 
InvalidRequestException(msg));
+        }
+
+        long now = System.currentTimeMillis();
+        ClientMetricsInstance clientInstance = 
getClientInstance(clientInstanceId, requestContext, now);
+
+        try {
+            // Validate the push request parameters for the client instance.
+            validatePushRequest(request, telemetryMaxBytes, clientInstance);
+        } catch (ApiException exception) {
+            clientInstance.lastKnownError(Errors.forException(exception));
+            return request.getErrorResponse(throttleMs, exception);
+        } finally {
+            // Update the client instance with the latest push request 
parameters.
+            clientInstance.terminating(request.data().terminating());
+            clientInstance.lastPushRequestEpoch(now);
+        }
+
+        // Push the metrics to the external client receiver plugin.
+        byte[] metrics = request.data().metrics();
+        if (metrics != null && metrics.length > 0) {
+            ClientMetricsReceiverPlugin.exportMetrics(requestContext, request);
+        }
+
+        clientInstance.lastKnownError(Errors.NONE);
+        return request.createResponse(throttleMs, Errors.NONE);
     }
 
     @Override
     public void close() throws IOException {
-        // TODO: Implement the close logic to close the client metrics manager.
+        // Do nothing for now.
+    }
+
+    private void updateLastSubscriptionUpdateEpoch() {
+        this.lastSubscriptionUpdateEpoch = System.currentTimeMillis();
+    }
+
+    private void updateClientSubscription(String subscriptionName, 
ClientMetricsConfigs configs) {
+        List<String> metrics = 
configs.getList(ClientMetricsConfigs.SUBSCRIPTION_METRICS);
+        int pushInterval = 
configs.getInt(ClientMetricsConfigs.PUSH_INTERVAL_MS);
+        List<String> clientMatchPattern = 
configs.getList(ClientMetricsConfigs.CLIENT_MATCH_PATTERN);
+
+        SubscriptionInfo newSubscription =
+            new SubscriptionInfo(subscriptionName, metrics, pushInterval,
+                
ClientMetricsConfigs.parseMatchingPatterns(clientMatchPattern));
+
+        subscriptionMap.put(subscriptionName, newSubscription);
+    }
+
+    private Uuid generateNewClientId() {
+        Uuid id = Uuid.randomUuid();
+        while (clientInstanceCache.get(id) != null) {
+            id = Uuid.randomUuid();
+        }
+        return id;
+    }
+
+    private ClientMetricsInstance getClientInstance(Uuid clientInstanceId, 
RequestContext requestContext,
+        long timestamp) {
+        // Check if null can be called on the cache. if can then we can avoid 
the method call.
+        ClientMetricsInstance clientInstance = 
clientInstanceCache.get(clientInstanceId);
+
+        if (clientInstance == null) {
+            // If the client instance is not present in the cache, then create 
a new client instance
+            // and update the cache. This can also happen when the telemetry 
request is received by
+            // the separate broker instance.
+            ClientMetricsInstanceMetadata instanceMetadata = new 
ClientMetricsInstanceMetadata(
+                clientInstanceId, requestContext);
+            clientInstance = 
createClientInstanceAndUpdateCache(clientInstanceId, instanceMetadata, 
timestamp);
+        } else if (clientInstance.subscriptionUpdateEpoch() < 
lastSubscriptionUpdateEpoch) {

Review Comment:
   Hmm, is skipping the `=` check enough? It seems that it's possible that a 
client picks up the first subscription, but misses the second subscription. If 
that happens, it's possible that a client won't pick up a newly matched 
subscription for a long time, right? Could we just use a counter and bump it up 
each time when there is a subscription update?



##########
core/src/main/java/kafka/server/ClientMetricsManager.java:
##########
@@ -30,17 +68,366 @@ public class ClientMetricsManager implements Closeable {
 
     private static final Logger log = 
LoggerFactory.getLogger(ClientMetricsManager.class);
     private static final ClientMetricsManager INSTANCE = new 
ClientMetricsManager();
+    private static final List<Byte> SUPPORTED_COMPRESSION_TYPES = 
Collections.unmodifiableList(
+        Arrays.asList(CompressionType.ZSTD.id, CompressionType.LZ4.id, 
CompressionType.GZIP.id, CompressionType.SNAPPY.id));
 
     public static ClientMetricsManager instance() {
         return INSTANCE;
     }
+    // Max cache size (16k active client connections per broker)
+    private static final int CM_CACHE_MAX_SIZE = 16384;
+    private final Cache<Uuid, ClientMetricsInstance> clientInstanceCache;
+    private final Map<String, SubscriptionInfo> subscriptionMap;
+
+    // The last subscription updated time is used to determine if the next 
telemetry request needs
+    // to re-evaluate the subscription id as per changes subscriptions.
+    private volatile long lastSubscriptionUpdateEpoch;
+
+    // Visible for testing
+    ClientMetricsManager() {
+        subscriptionMap = new ConcurrentHashMap<>();
+        clientInstanceCache = new SynchronizedCache<>(new 
LRUCache<>(CM_CACHE_MAX_SIZE));
+    }
 
     public void updateSubscription(String subscriptionName, Properties 
properties) {
-        // TODO: Implement the update logic to manage subscriptions.
+        // IncrementalAlterConfigs API will send empty configs when all the 
configs are deleted
+        // for respective subscription. In that case, we need to remove the 
subscription from the map.
+        if (properties.isEmpty()) {
+            // Remove the subscription from the map if it exists, else ignore 
the config update.
+            if (subscriptionMap.containsKey(subscriptionName)) {
+                log.info("Removing subscription [{}] from the subscription 
map", subscriptionName);
+                subscriptionMap.remove(subscriptionName);
+                updateLastSubscriptionUpdateEpoch();
+            }
+            return;
+        }
+
+        ClientMetricsConfigs configs = new ClientMetricsConfigs(properties);
+        updateClientSubscription(subscriptionName, configs);
+        /*
+         Update last subscription updated time to current time to indicate 
that there is a change
+         in the subscription. This will be used to determine if the next 
telemetry request needs
+         to re-evaluate the subscription id as per changes subscriptions.
+        */
+        updateLastSubscriptionUpdateEpoch();
+    }
+
+    public GetTelemetrySubscriptionsResponse 
processGetTelemetrySubscriptionRequest(
+        GetTelemetrySubscriptionsRequest request, int telemetryMaxBytes, 
RequestContext requestContext, int throttleMs) {
+
+        long now = System.currentTimeMillis();
+        Uuid clientInstanceId = 
Optional.ofNullable(request.data().clientInstanceId())
+            .filter(id -> !id.equals(Uuid.ZERO_UUID))
+            .orElse(generateNewClientId());
+
+        /*
+         Get the client instance from the cache or create a new one. If 
subscription has changed
+         since the last request, then the client instance will be 
re-evaluated. Validation of the
+         request will be done after the client instance is created. If client 
issued get telemetry
+         request prior to push interval, then the client should get a throttle 
error but if the
+         subscription has changed since the last request then the client 
should get the updated
+         subscription immediately.
+        */
+        ClientMetricsInstance clientInstance = 
clientInstance(clientInstanceId, requestContext, now);
+
+        try {
+            // Validate the get request parameters for the client instance.
+            validateGetRequest(request, clientInstance, now);
+        } catch (ApiException exception) {
+            return request.getErrorResponse(throttleMs, exception);
+        }
+
+        clientInstance.lastKnownError(Errors.NONE);
+        return createGetSubscriptionResponse(clientInstanceId, clientInstance, 
telemetryMaxBytes, throttleMs);
+    }
+
+    public PushTelemetryResponse 
processPushTelemetryRequest(PushTelemetryRequest request,
+        int telemetryMaxBytes, RequestContext requestContext, int throttleMs) {
+
+        Uuid clientInstanceId = request.data().clientInstanceId();
+        if (clientInstanceId == null || clientInstanceId == Uuid.ZERO_UUID) {
+            String msg = String.format("Invalid request from the client [%s], 
invalid client instance id",
+                clientInstanceId);
+            return request.getErrorResponse(throttleMs, new 
InvalidRequestException(msg));
+        }
+
+        long now = System.currentTimeMillis();
+        ClientMetricsInstance clientInstance = 
clientInstance(clientInstanceId, requestContext, now);
+
+        try {
+            // Validate the push request parameters for the client instance.
+            validatePushRequest(request, telemetryMaxBytes, clientInstance, 
now);
+        } catch (ApiException exception) {
+            log.debug("Error validating push telemetry request from client 
[{}]", clientInstanceId, exception);
+            clientInstance.lastKnownError(Errors.forException(exception));
+            return request.getErrorResponse(throttleMs, exception);
+        } finally {
+            // Update the client instance with the latest push request 
parameters.
+            clientInstance.terminating(request.data().terminating());
+        }
+
+        // Push the metrics to the external client receiver plugin.
+        byte[] metrics = request.data().metrics();
+        if (metrics != null && metrics.length > 0) {
+            try {
+                
ClientMetricsReceiverPlugin.instance().exportMetrics(requestContext, request);
+            } catch (Exception exception) {
+                clientInstance.lastKnownError(Errors.INVALID_RECORD);
+                return request.errorResponse(throttleMs, 
Errors.INVALID_RECORD);
+            }
+        }
+
+        clientInstance.lastKnownError(Errors.NONE);
+        return new PushTelemetryResponse(new 
PushTelemetryResponseData().setThrottleTimeMs(throttleMs));
     }
 
     @Override
     public void close() throws IOException {
-        // TODO: Implement the close logic to close the client metrics manager.
+        subscriptionMap.clear();
+    }
+
+    private synchronized void updateLastSubscriptionUpdateEpoch() {
+        this.lastSubscriptionUpdateEpoch = System.currentTimeMillis();
+    }
+
+    private void updateClientSubscription(String subscriptionName, 
ClientMetricsConfigs configs) {
+        List<String> metrics = 
configs.getList(ClientMetricsConfigs.SUBSCRIPTION_METRICS);
+        int pushInterval = 
configs.getInt(ClientMetricsConfigs.PUSH_INTERVAL_MS);
+        List<String> clientMatchPattern = 
configs.getList(ClientMetricsConfigs.CLIENT_MATCH_PATTERN);
+
+        SubscriptionInfo newSubscription =
+            new SubscriptionInfo(subscriptionName, metrics, pushInterval,
+                
ClientMetricsConfigs.parseMatchingPatterns(clientMatchPattern));
+
+        subscriptionMap.put(subscriptionName, newSubscription);
+    }
+
+    private Uuid generateNewClientId() {
+        Uuid id = Uuid.randomUuid();
+        while (clientInstanceCache.get(id) != null) {
+            id = Uuid.randomUuid();
+        }
+        return id;
+    }
+
+    private ClientMetricsInstance clientInstance(Uuid clientInstanceId, 
RequestContext requestContext,
+        long timestamp) {
+        ClientMetricsInstance clientInstance = 
clientInstanceCache.get(clientInstanceId);
+
+        if (clientInstance == null) {
+            /*
+             If the client instance is not present in the cache, then create a 
new client instance
+             and update the cache. This can also happen when the telemetry 
request is received by
+             the separate broker instance. Though cache is synchronized, but 
it is possible that
+             concurrent calls can create the same client instance. Hence, 
safeguard the client instance
+             creation with a double-checked lock to ensure that only one 
instance is created.
+            */
+            synchronized (this) {
+                clientInstance = clientInstanceCache.get(clientInstanceId);
+                if (clientInstance != null) {
+                    return clientInstance;
+                }
+
+                ClientMetricsInstanceMetadata instanceMetadata = new 
ClientMetricsInstanceMetadata(
+                    clientInstanceId, requestContext);
+                clientInstance = 
createClientInstanceAndUpdateCache(clientInstanceId, instanceMetadata, 
timestamp);
+            }
+        } else if (clientInstance.subscriptionUpdateEpoch() < 
lastSubscriptionUpdateEpoch) {
+            /*
+             If the last subscription update time for client instance is older 
than the subscription
+             updated time, then re-evaluate the subscription information for 
the client as per the
+             updated subscriptions. This is to ensure that the client instance 
is always in sync with
+             the latest subscription information. Though cache is 
synchronized, but it is possible that
+             concurrent calls can create the same client instance. Hence, 
safeguard the client instance
+             update with a double-checked lock to ensure that only one 
instance is created.
+            */
+            synchronized (this) {
+                clientInstance = clientInstanceCache.get(clientInstanceId);
+                if (clientInstance.subscriptionUpdateEpoch() >= 
lastSubscriptionUpdateEpoch) {
+                    return clientInstance;
+                }
+                clientInstance = 
createClientInstanceAndUpdateCache(clientInstanceId, 
clientInstance.instanceMetadata(), timestamp);
+            }
+        }
+
+        return clientInstance;
+    }
+
+    private ClientMetricsInstance createClientInstanceAndUpdateCache(Uuid 
clientInstanceId,
+        ClientMetricsInstanceMetadata instanceMetadata, long timestamp) {
+
+        ClientMetricsInstance clientInstance = 
createClientInstance(clientInstanceId, instanceMetadata,
+            timestamp);
+        clientInstanceCache.put(clientInstanceId, clientInstance);
+        return clientInstance;
+    }
+
+    private ClientMetricsInstance createClientInstance(Uuid clientInstanceId,
+        ClientMetricsInstanceMetadata instanceMetadata, long timestamp) {
+
+        int pushIntervalMs = ClientMetricsConfigs.DEFAULT_INTERVAL_MS;
+        // Keep a set of metrics to avoid duplicates in case of overlapping 
subscriptions.
+        Set<String> subscribedMetrics = new HashSet<>();
+        boolean allMetricsSubscribed = false;
+
+        for (SubscriptionInfo info : subscriptions()) {
+            if (instanceMetadata.isMatch(info.matchPattern())) {
+                allMetricsSubscribed = allMetricsSubscribed || 
info.metrics().contains(
+                    ClientMetricsConfigs.ALL_SUBSCRIBED_METRICS_CONFIG);
+                subscribedMetrics.addAll(info.metrics());
+                pushIntervalMs = Math.min(pushIntervalMs, info.intervalMs());
+            }
+        }
+
+        /*
+         If client matches with any subscription that has empty metrics 
string, then it means that client
+         is subscribed to all the metrics, so just send the empty string as 
the subscribed metrics.
+        */
+        if (allMetricsSubscribed) {
+            subscribedMetrics.clear();
+            // Add an empty string to indicate that all metrics are subscribed.
+            subscribedMetrics.add(ClientMetricsConfigs.ALL_SUBSCRIBED_METRICS);
+        }
+
+        int subscriptionId = computeSubscriptionId(subscribedMetrics, 
pushIntervalMs, clientInstanceId);
+
+        return new ClientMetricsInstance(clientInstanceId, instanceMetadata, 
subscriptionId, timestamp,
+            subscribedMetrics, pushIntervalMs);
+    }
+
+    /**
+     * Computes the SubscriptionId as a unique identifier for a client 
instance's subscription set,
+     * the id is generated by calculating a CRC32 of the configured metrics 
subscriptions including
+     * the PushIntervalMs, XORed with the ClientInstanceId.
+     */
+    private int computeSubscriptionId(Set<String> metrics, int pushIntervalMs, 
Uuid clientInstanceId) {
+        CRC32 crc = new CRC32();
+        byte[] metricsBytes = (metrics.toString() + 
pushIntervalMs).getBytes(StandardCharsets.UTF_8);
+        crc.update(ByteBuffer.wrap(metricsBytes));
+        return (int) crc.getValue() ^ clientInstanceId.hashCode();
+    }
+
+    private GetTelemetrySubscriptionsResponse 
createGetSubscriptionResponse(Uuid clientInstanceId,
+        ClientMetricsInstance clientInstance, int telemetryMaxBytes, int 
throttleMs) {
+
+        GetTelemetrySubscriptionsResponseData data = new 
GetTelemetrySubscriptionsResponseData()
+            .setClientInstanceId(clientInstanceId)
+            .setSubscriptionId(clientInstance.subscriptionId())
+            .setRequestedMetrics(new ArrayList<>(clientInstance.metrics()))
+            .setAcceptedCompressionTypes(SUPPORTED_COMPRESSION_TYPES)
+            .setPushIntervalMs(clientInstance.pushIntervalMs())
+            .setTelemetryMaxBytes(telemetryMaxBytes)
+            .setDeltaTemporality(true)
+            .setErrorCode(Errors.NONE.code())
+            .setThrottleTimeMs(throttleMs);
+
+        return new GetTelemetrySubscriptionsResponse(data);
+    }
+
+    private void validateGetRequest(GetTelemetrySubscriptionsRequest request,
+        ClientMetricsInstance clientInstance, long timestamp) {
+
+        if (!clientInstance.maybeUpdateGetRequestEpoch(timestamp) && 
(clientInstance.lastKnownError() != Errors.UNKNOWN_SUBSCRIPTION_ID

Review Comment:
   Why only testing these two errors? What about other errors like 
`InvalidRequest`?



##########
core/src/test/java/kafka/server/ClientMetricsManagerTest.java:
##########
@@ -0,0 +1,921 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package kafka.server;
+
+import kafka.metrics.ClientMetricsConfigs;
+import kafka.metrics.ClientMetricsInstance;
+import kafka.metrics.ClientMetricsTestUtils;
+import kafka.server.ClientMetricsManager.SubscriptionInfo;
+
+import org.apache.kafka.common.Uuid;
+import org.apache.kafka.common.message.GetTelemetrySubscriptionsRequestData;
+import org.apache.kafka.common.message.PushTelemetryRequestData;
+import org.apache.kafka.common.protocol.Errors;
+import org.apache.kafka.common.record.CompressionType;
+import org.apache.kafka.common.requests.GetTelemetrySubscriptionsRequest;
+import org.apache.kafka.common.requests.GetTelemetrySubscriptionsResponse;
+import org.apache.kafka.common.requests.PushTelemetryRequest;
+import org.apache.kafka.common.requests.PushTelemetryRequest.Builder;
+import org.apache.kafka.common.requests.PushTelemetryResponse;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import java.net.UnknownHostException;
+import java.nio.charset.StandardCharsets;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.List;
+import java.util.Properties;
+import java.util.Set;
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.TimeUnit;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+public class ClientMetricsManagerTest {
+
+    private ClientMetricsManager clientMetricsManager;
+
+    @BeforeEach
+    public void setUp() {
+        clientMetricsManager = new ClientMetricsManager();
+    }
+
+    @Test
+    public void testUpdateSubscription() {
+        assertTrue(clientMetricsManager.subscriptions().isEmpty());
+
+        long previousEpoch = 
clientMetricsManager.lastSubscriptionUpdateEpoch();
+        clientMetricsManager.updateSubscription("sub-1", 
ClientMetricsTestUtils.defaultProperties());
+
+        assertEquals(1, clientMetricsManager.subscriptions().size());
+        assertNotNull(clientMetricsManager.subscriptionInfo("sub-1"));
+
+        SubscriptionInfo subscriptionInfo = 
clientMetricsManager.subscriptionInfo("sub-1");
+        Set<String> metrics = subscriptionInfo.metrics();
+
+        // Validate metrics.
+        assertEquals(ClientMetricsTestUtils.DEFAULT_METRICS.split(",").length, 
metrics.size());
+        
Arrays.stream(ClientMetricsTestUtils.DEFAULT_METRICS.split(",")).forEach(metric 
->
+            assertTrue(metrics.contains(metric)));
+        // Validate push interval.
+        
assertEquals(ClientMetricsTestUtils.defaultProperties().getProperty(ClientMetricsConfigs.PUSH_INTERVAL_MS),
+            String.valueOf(subscriptionInfo.intervalMs()));
+
+        // Validate match patterns.
+        
assertEquals(ClientMetricsTestUtils.DEFAULT_CLIENT_MATCH_PATTERNS.size(),
+            subscriptionInfo.matchPattern().size());
+        ClientMetricsTestUtils.DEFAULT_CLIENT_MATCH_PATTERNS.forEach(pattern 
-> {
+            String[] split = pattern.split("=");
+            assertTrue(subscriptionInfo.matchPattern().containsKey(split[0]));
+            assertEquals(split[1], 
subscriptionInfo.matchPattern().get(split[0]).pattern());
+        });
+        assertTrue(clientMetricsManager.lastSubscriptionUpdateEpoch() > 
previousEpoch);
+    }
+
+    @Test
+    public void testUpdateSubscriptionWithEmptyProperties() {
+        assertTrue(clientMetricsManager.subscriptions().isEmpty());
+        long previousEpoch = 
clientMetricsManager.lastSubscriptionUpdateEpoch();
+        clientMetricsManager.updateSubscription("sub-1", new Properties());
+        // No subscription should be added as the properties are empty.
+        assertEquals(0, clientMetricsManager.subscriptions().size());
+        assertEquals(previousEpoch, 
clientMetricsManager.lastSubscriptionUpdateEpoch());
+    }
+
+    @Test
+    public void testUpdateSubscriptionWithNullProperties() {
+        assertTrue(clientMetricsManager.subscriptions().isEmpty());
+        long previousEpoch = 
clientMetricsManager.lastSubscriptionUpdateEpoch();
+        // Properties shouldn't be passed as null.
+        assertThrows(NullPointerException.class, () ->
+            clientMetricsManager.updateSubscription("sub-1", null));
+        assertEquals(0, clientMetricsManager.subscriptions().size());
+        assertEquals(previousEpoch, 
clientMetricsManager.lastSubscriptionUpdateEpoch());
+    }
+
+    @Test
+    public void testUpdateSubscriptionWithInvalidMetricsProperties() {
+        assertTrue(clientMetricsManager.subscriptions().isEmpty());
+
+        long previousEpoch = 
clientMetricsManager.lastSubscriptionUpdateEpoch();
+        Properties properties = new Properties();
+        properties.put("random", "random");
+        clientMetricsManager.updateSubscription("sub-1", properties);
+
+        // Undefined keys are ignored.
+        assertEquals(1, clientMetricsManager.subscriptions().size());
+        assertNotNull(clientMetricsManager.subscriptionInfo("sub-1"));
+        // Config def default properties.
+        
assertTrue(clientMetricsManager.subscriptionInfo("sub-1").metrics().isEmpty());
+        assertEquals(ClientMetricsConfigs.DEFAULT_INTERVAL_MS, 
clientMetricsManager.subscriptionInfo("sub-1").intervalMs());
+        
assertTrue(clientMetricsManager.subscriptionInfo("sub-1").matchPattern().isEmpty());
+        assertTrue(clientMetricsManager.lastSubscriptionUpdateEpoch() > 
previousEpoch);
+    }
+
+    @Test
+    public void testUpdateSubscriptionWithPropertiesDeletion() throws 
InterruptedException {
+        assertTrue(clientMetricsManager.subscriptions().isEmpty());
+
+        long previousEpoch = 
clientMetricsManager.lastSubscriptionUpdateEpoch();
+        Properties properties = new Properties();
+        properties.put("interval.ms", "100");
+        clientMetricsManager.updateSubscription("sub-1", properties);
+        assertEquals(1, clientMetricsManager.subscriptions().size());
+        assertNotNull(clientMetricsManager.subscriptionInfo("sub-1"));
+        assertTrue(clientMetricsManager.lastSubscriptionUpdateEpoch() > 
previousEpoch);
+
+        previousEpoch = clientMetricsManager.lastSubscriptionUpdateEpoch();
+        Thread.sleep(1);
+        clientMetricsManager.updateSubscription("sub-1", new Properties());
+        // Subscription should be removed as all properties are removed.
+        assertEquals(0, clientMetricsManager.subscriptions().size());
+        assertTrue(clientMetricsManager.lastSubscriptionUpdateEpoch() > 
previousEpoch);
+    }
+
+    @Test
+    public void testGetTelemetry() throws UnknownHostException {
+        clientMetricsManager.updateSubscription("sub-1", 
ClientMetricsTestUtils.defaultProperties());
+        assertEquals(1, clientMetricsManager.subscriptions().size());
+
+        GetTelemetrySubscriptionsRequest request = new 
GetTelemetrySubscriptionsRequest.Builder(
+            new GetTelemetrySubscriptionsRequestData()).build();
+
+        GetTelemetrySubscriptionsResponse response = 
clientMetricsManager.processGetTelemetrySubscriptionRequest(
+            request, 100, ClientMetricsTestUtils.requestContext(), 0);
+
+        assertNotNull(response.data().clientInstanceId());
+        assertTrue(response.data().subscriptionId() != 0);
+
+        assertEquals(ClientMetricsTestUtils.DEFAULT_METRICS.split(",").length, 
response.data().requestedMetrics().size());
+        
Arrays.stream(ClientMetricsTestUtils.DEFAULT_METRICS.split(",")).forEach(metric 
->
+            assertTrue(response.data().requestedMetrics().contains(metric)));
+
+        assertEquals(4, response.data().acceptedCompressionTypes().size());
+        // validate compression types order.
+        assertEquals(CompressionType.ZSTD.id, 
response.data().acceptedCompressionTypes().get(0));
+        assertEquals(CompressionType.LZ4.id, 
response.data().acceptedCompressionTypes().get(1));
+        assertEquals(CompressionType.GZIP.id, 
response.data().acceptedCompressionTypes().get(2));
+        assertEquals(CompressionType.SNAPPY.id, 
response.data().acceptedCompressionTypes().get(3));
+        assertEquals(ClientMetricsTestUtils.DEFAULT_PUSH_INTERVAL_MS, 
response.data().pushIntervalMs());
+        assertTrue(response.data().deltaTemporality());
+        assertEquals(100, response.data().telemetryMaxBytes());
+        assertEquals(Errors.NONE, response.error());
+
+        ClientMetricsInstance instance = 
clientMetricsManager.clientInstance(response.data().clientInstanceId());
+        assertNotNull(instance);
+        assertEquals(Errors.NONE, instance.lastKnownError());
+    }
+
+    @Test
+    public void testGetTelemetryWithoutSubscription() throws 
UnknownHostException {
+        assertTrue(clientMetricsManager.subscriptions().isEmpty());
+
+        GetTelemetrySubscriptionsRequest request = new 
GetTelemetrySubscriptionsRequest.Builder(
+            new GetTelemetrySubscriptionsRequestData()).build();
+
+        GetTelemetrySubscriptionsResponse response = 
clientMetricsManager.processGetTelemetrySubscriptionRequest(
+            request, 1024 * 1024, ClientMetricsTestUtils.requestContext(), 0);
+
+        assertNotNull(response.data().clientInstanceId());
+        assertTrue(response.data().subscriptionId() != 0);
+        assertTrue(response.data().requestedMetrics().isEmpty());
+        assertEquals(4, response.data().acceptedCompressionTypes().size());
+        assertEquals(ClientMetricsConfigs.DEFAULT_INTERVAL_MS, 
response.data().pushIntervalMs());
+        assertTrue(response.data().deltaTemporality());
+        assertEquals(1024 * 1024, response.data().telemetryMaxBytes());
+        assertEquals(Errors.NONE, response.error());
+
+        ClientMetricsInstance instance = 
clientMetricsManager.clientInstance(response.data().clientInstanceId());
+        assertNotNull(instance);
+        assertEquals(Errors.NONE, instance.lastKnownError());
+    }
+
+    @Test
+    public void testGetTelemetryAllMetricSubscribedSubscription() throws 
UnknownHostException {
+        clientMetricsManager.updateSubscription("sub-1", 
ClientMetricsTestUtils.defaultProperties());
+        Properties properties = new Properties();
+        properties.put("metrics", 
ClientMetricsConfigs.ALL_SUBSCRIBED_METRICS_CONFIG);
+        clientMetricsManager.updateSubscription("sub-2", properties);
+
+        assertEquals(2, clientMetricsManager.subscriptions().size());
+
+        GetTelemetrySubscriptionsRequest request = new 
GetTelemetrySubscriptionsRequest.Builder(
+            new GetTelemetrySubscriptionsRequestData()).build();
+
+        GetTelemetrySubscriptionsResponse response = 
clientMetricsManager.processGetTelemetrySubscriptionRequest(
+            request, 100, ClientMetricsTestUtils.requestContext(), 0);
+
+        assertNotNull(response.data().clientInstanceId());
+        assertTrue(response.data().subscriptionId() != 0);
+
+        assertEquals(1, response.data().requestedMetrics().size());
+        
assertTrue(response.data().requestedMetrics().contains(ClientMetricsConfigs.ALL_SUBSCRIBED_METRICS));
+
+        assertEquals(4, response.data().acceptedCompressionTypes().size());
+        assertEquals(ClientMetricsTestUtils.DEFAULT_PUSH_INTERVAL_MS, 
response.data().pushIntervalMs());
+        assertTrue(response.data().deltaTemporality());
+        assertEquals(100, response.data().telemetryMaxBytes());
+        assertEquals(Errors.NONE, response.error());
+
+        ClientMetricsInstance instance = 
clientMetricsManager.clientInstance(response.data().clientInstanceId());
+        assertNotNull(instance);
+        assertEquals(Errors.NONE, instance.lastKnownError());
+    }
+
+    @Test
+    public void testGetTelemetrySameClientImmediateRetryFail() throws 
UnknownHostException {
+        GetTelemetrySubscriptionsRequest request = new 
GetTelemetrySubscriptionsRequest.Builder(
+            new GetTelemetrySubscriptionsRequestData()).build();
+
+        GetTelemetrySubscriptionsResponse response = 
clientMetricsManager.processGetTelemetrySubscriptionRequest(
+            request, 100, ClientMetricsTestUtils.requestContext(), 0);
+
+        Uuid clientInstanceId = response.data().clientInstanceId();
+        assertNotNull(clientInstanceId);
+        assertEquals(Errors.NONE, response.error());
+
+        request = new GetTelemetrySubscriptionsRequest.Builder(
+            new 
GetTelemetrySubscriptionsRequestData().setClientInstanceId(clientInstanceId)).build();
+        response = clientMetricsManager.processGetTelemetrySubscriptionRequest(
+            request, 100, ClientMetricsTestUtils.requestContext(), 0);
+
+        assertEquals(Errors.THROTTLING_QUOTA_EXCEEDED, response.error());
+    }
+
+    @Test
+    public void testGetTelemetrySameClientImmediateRetryAfterPushFail() throws 
UnknownHostException {
+        GetTelemetrySubscriptionsRequest request = new 
GetTelemetrySubscriptionsRequest.Builder(
+            new GetTelemetrySubscriptionsRequestData()).build();
+
+        GetTelemetrySubscriptionsResponse response = 
clientMetricsManager.processGetTelemetrySubscriptionRequest(
+            request, 100, ClientMetricsTestUtils.requestContext(), 0);
+
+        Uuid clientInstanceId = response.data().clientInstanceId();
+        assertNotNull(clientInstanceId);
+        assertEquals(Errors.NONE, response.error());
+
+        // Create new client metrics manager which simulates a new server as 
it will not have any
+        // last request information but request should succeed as subscription 
id should match
+        // the one with new client instance.
+
+        ClientMetricsManager newClientMetricsManager = new 
ClientMetricsManager();
+
+        PushTelemetryRequest pushRequest = new Builder(
+            new PushTelemetryRequestData()
+                .setClientInstanceId(response.data().clientInstanceId())
+                .setSubscriptionId(response.data().subscriptionId())
+                .setCompressionType(CompressionType.NONE.id)
+                
.setMetrics("test-data".getBytes(StandardCharsets.UTF_8))).build();
+
+        PushTelemetryResponse pushResponse = 
newClientMetricsManager.processPushTelemetryRequest(
+            pushRequest, 100, ClientMetricsTestUtils.requestContext(), 0);
+
+        assertEquals(Errors.NONE, pushResponse.error());
+
+        request = new GetTelemetrySubscriptionsRequest.Builder(
+            new 
GetTelemetrySubscriptionsRequestData().setClientInstanceId(clientInstanceId)).build();
+
+        response = 
newClientMetricsManager.processGetTelemetrySubscriptionRequest(
+            request, 100, ClientMetricsTestUtils.requestContext(), 0);
+
+        assertEquals(Errors.THROTTLING_QUOTA_EXCEEDED, response.error());
+    }
+
+    @Test
+    public void testGetTelemetryUpdateSubscription() throws Exception {
+        clientMetricsManager.updateSubscription("sub-1", 
ClientMetricsTestUtils.defaultProperties());
+        assertEquals(1, clientMetricsManager.subscriptions().size());
+
+        GetTelemetrySubscriptionsRequest request = new 
GetTelemetrySubscriptionsRequest.Builder(
+            new GetTelemetrySubscriptionsRequestData()).build();
+
+        GetTelemetrySubscriptionsResponse response = 
clientMetricsManager.processGetTelemetrySubscriptionRequest(
+            request, 100, ClientMetricsTestUtils.requestContext(), 0);
+
+        Uuid clientInstanceId = response.data().clientInstanceId();
+        int subscriptionId = response.data().subscriptionId();
+        assertNotNull(clientInstanceId);
+        assertTrue(subscriptionId != 0);
+        assertEquals(Errors.NONE, response.error());
+
+        // Wait for subscription time to be greater from last client instance 
time.
+        Thread.sleep(10);
+
+        // Update subscription
+        Properties properties = new Properties();
+        properties.put("metrics", 
ClientMetricsConfigs.ALL_SUBSCRIBED_METRICS_CONFIG);
+        clientMetricsManager.updateSubscription("sub-2", properties);
+        assertEquals(2, clientMetricsManager.subscriptions().size());
+
+        request = new GetTelemetrySubscriptionsRequest.Builder(
+            new 
GetTelemetrySubscriptionsRequestData().setClientInstanceId(clientInstanceId)).build();
+        response = clientMetricsManager.processGetTelemetrySubscriptionRequest(
+            request, 100, ClientMetricsTestUtils.requestContext(), 0);
+
+        // No throttle error as the subscription has changed.
+        assertEquals(Errors.NONE, response.error());
+        // Subscription id updated in next request
+        assertTrue(subscriptionId != response.data().subscriptionId());
+    }
+
+    @Test
+    public void testGetTelemetryConcurrentRequestNewClientInstance() throws 
InterruptedException {
+        GetTelemetrySubscriptionsRequest request = new 
GetTelemetrySubscriptionsRequest.Builder(
+            new 
GetTelemetrySubscriptionsRequestData().setClientInstanceId(Uuid.randomUuid())).build();
+
+        CountDownLatch lock = new CountDownLatch(2);
+        List<GetTelemetrySubscriptionsResponse> responses = 
Collections.synchronizedList(new ArrayList<>());
+
+        Thread thread = new Thread(() -> {
+            try {
+                GetTelemetrySubscriptionsResponse response = 
clientMetricsManager.processGetTelemetrySubscriptionRequest(
+                    request, 100, ClientMetricsTestUtils.requestContext(), 0);
+
+                responses.add(response);
+                lock.countDown();
+            } catch (UnknownHostException e) {
+                e.printStackTrace();

Review Comment:
   We should use logging instead of print. Ditto in other places.



##########
clients/src/main/java/org/apache/kafka/common/requests/PushTelemetryRequest.java:
##########
@@ -71,6 +73,31 @@ public PushTelemetryRequestData data() {
         return data;
     }
 
+    public PushTelemetryResponse createResponse(int throttleTimeMs, Errors 
errors) {
+        PushTelemetryResponseData responseData = new 
PushTelemetryResponseData();
+        responseData.setErrorCode(errors.code());
+        responseData.setThrottleTimeMs(throttleTimeMs);

Review Comment:
   The following is my understanding. There are two types of throttling.
   
   1. Generic request throttling based on % of CPU a client uses on the broker. 
This applies to any request, including `PushTelemetry`. In this case, the error 
code is none and the `throttleTimeMs` field is set. The client will mute the 
channel for `throttleTimeMs` before sending future requests.
   2. `PushTelemetry` specific throttling because PushTelemetry is sent too 
frequently. In this case, we should set THROTTLING_QUOTA_EXCEEDED as the error 
code and avoid setting the `throttleTimeMs` field since we don't want to the 
client to mute the channel for all requests.



##########
core/src/main/java/kafka/metrics/ClientMetricsConfigs.java:
##########
@@ -80,6 +82,11 @@ public class ClientMetricsConfigs {
     public static final String CLIENT_SOURCE_ADDRESS = "client_source_address";
     public static final String CLIENT_SOURCE_PORT = "client_source_port";
 
+    // Empty string in client-metrics resource configs indicates that all the 
metrics are subscribed.
+    public static final String ALL_SUBSCRIBED_METRICS_CONFIG = "\"\"";

Review Comment:
   Hmm, this can be confusing. Would it be better to change the KIP to use sth 
like `*` to represent all metrics? cc @AndrewJSchofield 



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