### Motivation
The call to `serverStats().getNumAliveClientConnections()` inside ZK server is
throwing NPE in some cases where the `ServerCnxnFactory` is null.
```
22:07:04.269 [qtp33419717-17] WARN org.eclipse.jetty.servlet.ServletHandler -
/metrics
java.lang.NullPointerException: null
at
org.apache.zookeeper.server.ZooKeeperServer.getNumAliveConnections(ZooKeeperServer.java:883)
~[org.apache.pulsar-pulsar-broker-2.2.0-streamlio-4.jar:2.2.0-streamlio-4]
at
org.apache.zookeeper.server.ServerStats.getNumAliveClientConnections(ServerStats.java:90)
~[org.apache.pulsar-pulsar-broker-2.2.0-streamlio-4.jar:2.2.0-streamlio-4]
at
org.apache.pulsar.zookeeper.ZooKeeperServerAspect$3.get(ZooKeeperServerAspect.java:74)
~[org.apache.pulsar-pulsar-zookeeper-2.2.0-streamlio-4.jar:2.2.0-streamlio-4]
at io.prometheus.client.Gauge.collect(Gauge.java:295)
~[io.prometheus-simpleclient-0.0.23.jar:?]
at
io.prometheus.client.CollectorRegistry$MetricFamilySamplesEnumeration.findNextElement(CollectorRegistry.java:180)
~[io.prometheus-simpleclient-0.0.23.jar:?]
at
io.prometheus.client.CollectorRegistry$MetricFamilySamplesEnumeration.nextElement(CollectorRegistry.java:213)
~[io.prometheus-simpleclient-0.0.23.jar:?]
at
io.prometheus.client.CollectorRegistry$MetricFamilySamplesEnumeration.nextElement(CollectorRegistry.java:134)
~[io.prometheus-simpleclient-0.0.23.jar:?]
at
io.prometheus.client.exporter.common.TextFormat.write004(TextFormat.java:22)
~[io.prometheus-simpleclient_common-0.0.23.jar:?]
at
io.prometheus.client.exporter.MetricsServlet.doGet(MetricsServlet.java:43)
~[io.prometheus-simpleclient_servlet-0.0.23.jar:?]
```
This also break the metrics output since it won't print any more metrics after
this exception.
### Modifications
Bypass the `ServerStats` object and directly get the info on the
`ServerCnxnFactory`, validating that it's not null.
[ Full content available at: https://github.com/apache/pulsar/pull/2710 ]
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