igiguere commented on code in PR #4057:
URL: https://github.com/apache/solr/pull/4057#discussion_r2708392860


##########
solr/core/src/java/org/apache/solr/metrics/MetricsUtil.java:
##########
@@ -0,0 +1,186 @@
+/*
+ * 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 org.apache.solr.metrics;
+
+import io.prometheus.metrics.model.snapshots.CounterSnapshot;
+import io.prometheus.metrics.model.snapshots.GaugeSnapshot;
+import io.prometheus.metrics.model.snapshots.HistogramSnapshot;
+import io.prometheus.metrics.model.snapshots.InfoSnapshot;
+import io.prometheus.metrics.model.snapshots.MetricSnapshot;
+import io.prometheus.metrics.model.snapshots.MetricSnapshots;
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+import java.util.SortedMap;
+import java.util.TreeMap;
+import org.apache.solr.common.params.SolrParams;
+import org.apache.solr.common.util.StrUtils;
+
+/** Utility methods for Metrics */
+public class MetricsUtil {
+
+  public static final String PROMETHEUS_METRICS_WT = "prometheus";
+  public static final String OPEN_METRICS_WT = "openmetrics";
+
+  public static final String CATEGORY_PARAM = "category";
+  public static final String CORE_PARAM = "core";
+  public static final String COLLECTION_PARAM = "collection";
+  public static final String SHARD_PARAM = "shard";
+  public static final String REPLICA_TYPE_PARAM = "replica_type";
+  public static final String METRIC_NAME_PARAM = "name";
+  private static final Set<String> labelFilterKeys =
+      Set.of(CATEGORY_PARAM, CORE_PARAM, COLLECTION_PARAM, SHARD_PARAM, 
REPLICA_TYPE_PARAM);
+
+  /**
+   * Merge a collection of individual {@link MetricSnapshot} instances into 
one {@link
+   * MetricSnapshots}. This is necessary because we create a {@link
+   * io.opentelemetry.sdk.metrics.SdkMeterProvider} per Solr core resulting in 
duplicate metric
+   * names across cores which is an illegal format if under the same 
prometheus grouping.
+   */
+  public static MetricSnapshots mergeSnapshots(List<MetricSnapshot> snapshots) 
{
+    Map<String, CounterSnapshot.Builder> counterSnapshotMap = new HashMap<>();
+    Map<String, GaugeSnapshot.Builder> gaugeSnapshotMap = new HashMap<>();
+    Map<String, HistogramSnapshot.Builder> histogramSnapshotMap = new 
HashMap<>();
+    InfoSnapshot otelInfoSnapshots = null;
+
+    for (MetricSnapshot snapshot : snapshots) {
+      String metricName = snapshot.getMetadata().getPrometheusName();
+
+      switch (snapshot) {
+        case CounterSnapshot counterSnapshot -> {
+          CounterSnapshot.Builder builder =
+              counterSnapshotMap.computeIfAbsent(
+                  metricName,
+                  k -> {
+                    var base =
+                        CounterSnapshot.builder()
+                            .name(counterSnapshot.getMetadata().getName())
+                            .help(counterSnapshot.getMetadata().getHelp());
+                    return counterSnapshot.getMetadata().hasUnit()
+                        ? base.unit(counterSnapshot.getMetadata().getUnit())
+                        : base;
+                  });
+          counterSnapshot.getDataPoints().forEach(builder::dataPoint);
+        }
+        case GaugeSnapshot gaugeSnapshot -> {
+          GaugeSnapshot.Builder builder =
+              gaugeSnapshotMap.computeIfAbsent(
+                  metricName,
+                  k -> {
+                    var base =
+                        GaugeSnapshot.builder()
+                            .name(gaugeSnapshot.getMetadata().getName())
+                            .help(gaugeSnapshot.getMetadata().getHelp());
+                    return gaugeSnapshot.getMetadata().hasUnit()
+                        ? base.unit(gaugeSnapshot.getMetadata().getUnit())
+                        : base;
+                  });
+          gaugeSnapshot.getDataPoints().forEach(builder::dataPoint);
+        }
+        case HistogramSnapshot histogramSnapshot -> {
+          HistogramSnapshot.Builder builder =
+              histogramSnapshotMap.computeIfAbsent(
+                  metricName,
+                  k -> {
+                    var base =
+                        HistogramSnapshot.builder()
+                            .name(histogramSnapshot.getMetadata().getName())
+                            .help(histogramSnapshot.getMetadata().getHelp());
+                    return histogramSnapshot.getMetadata().hasUnit()
+                        ? base.unit(histogramSnapshot.getMetadata().getUnit())
+                        : base;
+                  });
+          histogramSnapshot.getDataPoints().forEach(builder::dataPoint);
+        }
+        case InfoSnapshot infoSnapshot -> {
+          // InfoSnapshot is a special case in that each SdkMeterProvider will 
create a duplicate
+          // metric called target_info containing OTEL SDK metadata. Only one 
of these need to be
+          // kept
+          if (otelInfoSnapshots == null)
+            otelInfoSnapshots =
+                new InfoSnapshot(infoSnapshot.getMetadata(), 
infoSnapshot.getDataPoints());
+        }
+        default -> {
+          // Handle unexpected snapshot types gracefully
+        }
+      }
+    }
+
+    MetricSnapshots.Builder snapshotsBuilder = MetricSnapshots.builder();
+    counterSnapshotMap.values().forEach(b -> 
snapshotsBuilder.metricSnapshot(b.build()));
+    gaugeSnapshotMap.values().forEach(b -> 
snapshotsBuilder.metricSnapshot(b.build()));
+    histogramSnapshotMap.values().forEach(b -> 
snapshotsBuilder.metricSnapshot(b.build()));
+    if (otelInfoSnapshots != null) 
snapshotsBuilder.metricSnapshot(otelInfoSnapshots);
+    return snapshotsBuilder.build();
+  }
+
+  /** Gather label filters */
+  public static SortedMap<String, Set<String>> labelFilters(SolrParams params) 
{
+    SortedMap<String, Set<String>> labelFilters = new TreeMap<>();
+    labelFilterKeys.forEach(
+        (paramName) -> {
+          Set<String> filterValues = readParamsAsSet(params, paramName);
+          if (!filterValues.isEmpty()) {
+            labelFilters.put(paramName, filterValues);
+          }
+        });
+
+    return labelFilters;
+  }
+
+  /** Add label filters to the filters map */
+  public static void addLabelFilters(String value, Map<String, Set<String>> 
filters) {
+    labelFilterKeys.forEach(
+        (paramName) -> {
+          Set<String> filterValues = paramValueAsSet(value);
+          if (!filterValues.isEmpty()) {
+            filters.put(paramName, filterValues);
+          }
+        });
+  }
+
+  /** Split the coma-separated param values into a set */
+  public static Set<String> paramValueAsSet(String paramValue) {
+    String[] values = paramValue.split(",");
+    List<String> valuesSet = new ArrayList<>();
+    for (String value : values) {
+      valuesSet.add(value);
+    }
+    return Set.copyOf(valuesSet);
+  }
+
+  /**
+   * Read Solr parameters as a Set.
+   *
+   * <p>Could probably be moved to a more generic utility class, but only used 
in MetricsHandler and
+   * GetMetrics resource.
+   */
+  public static Set<String> readParamsAsSet(SolrParams params, String 
paramName) {
+    String[] paramValues = params.getParams(paramName);
+    if (paramValues == null || paramValues.length == 0) {
+      return Set.of();
+    }
+
+    List<String> paramSet = new ArrayList<>();

Review Comment:
   Pre-exisiting code, move here, but, might as well improve it.



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