Add client-side metrics to the ZooKeeper Java client

[arkmish]

Description

Adds client-side metrics to the ZooKeeper Java client, built on the existing pluggable MetricsProvider framework (which was always intended for client use, per ZOOKEEPER-3103). Until now only the server emitted metrics via ServerMetrics; the Java client emitted none.

• What: a new ClientMetrics singleton (mirroring ServerMetrics) plus instrumentation in ClientCnxn. It records per-operation request latency and count, unsuccessful-request counts labelled by KeeperException.Code, ping round-trip time, connect latency, and connection established/loss/reconnect and session-expired counters.
• Why: give operators visibility into client-observed behavior (op traffic, latency, errors, connection/session churn) — aligning with the open upstream requests ZOOKEEPER-4983 / ZOOKEEPER-4863 / ZOOKEEPER-4841.
• How: metrics are emitted from a single request funnel (queuePacket/finishPacket) and the SendThread lifecycle. The feature is opt-in: the default provider is NullMetricsProvider (zero overhead), enabled via new zookeeper.clientMetricsProvider.className / zookeeper.clientMetricsProvider.* client properties on ZKClientConfig. The provider is a per-JVM singleton shared by all ZooKeeper instances so a real provider is not booted twice (e.g. Prometheus port binding).

Tests

• Local code review completed

The following tests are written for this issue:

• ClientMetricsTest (unit — singleton, per-op latency/count, error-by-code, lifecycle counters/summaries)
• ZKClientConfigMetricsTest (unit — provider class-name default/override and metricsProvider.* passthrough)
• ClientMetricsIntegrationTest (integration — real client↔server; asserts create/getData counts, NONODE error count, and connection-established are emitted)

The following is the result of the "mvn test" command on the appropriate module (zookeeper-server):

mvn -pl zookeeper-server -am test -Dtest='ClientMetricsTest,ZKClientConfigMetricsTest,ClientMetricsIntegrationTest'
Tests run: 9, Failures: 0, Errors: 0, Skipped: 0
BUILD SUCCESS

Existing ZKClientConfigTest (6) still passes. (Note: JMX-harness integration tests are broken in this local environment under Java 25/RMI, unrelated to this change; the metrics integration test is self-contained and avoids that harness.)

Changes that Break Backward Compatibility (Optional)

None. The feature is purely additive and opt-in: with no configuration the client uses NullMetricsProvider, so existing behavior and overhead are unchanged.

Documentation (Optional)

Adds a "Client Metrics" section to zookeeper-docs/.../zookeeperMonitor.md documenting how to enable the provider, the per-JVM semantics, and the emitted metrics.

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