Close pool gauge registrations on shutdown

[KirillKurdyukov]

Summary

Release observable gauge callbacks when table/query session pools close, following the C# reporter registration/disposal lifecycle.

• New Meter.registerLongGauge returns a closeable MetricRegistration.
• OpenTelemetryMeter wraps the native gauge in a private GaugeRegistration with a final field and delegates close() directly.
• Shared PoolMetrics owns its four gauge registrations; both table and query pool owners close them.

Gauge names, units, values, counter paths and session/queue behavior remain unchanged. No Topic changes are included. This is the prerequisite for rebasing and correcting Topic gauges in #734 after this PR is merged.

Existing void Meter.createLongGauge remains source/binary compatible and is marked @Deprecated. The new method has a default implementation that calls the legacy override and returns MetricRegistration.NOOP. Backends override registerLongGauge to support callback removal; existing custom backends keep their previous lifecycle.

Validation

Java21 SDK clean install passed 69 tests: core6, table42, query21; no failures, errors or skips. This includes legacy API compatibility and callback removal.

Current JDBC source compiled all 89 production and 55 test files against the local SDK2.4.12-SNAPSHOT. Its 89 settings tests passed without failures/errors/skips, including the unchanged legacy BadCustomMeter implementation.

JAVA_HOME=$(/usr/libexec/java_home -v 21) mvn -B -pl query -am \
  '-Dtest=OpenTelemetryMeterTest,PoolMetricsTest,SessionPoolTest,WaitingQueueTest' \
  -Dsurefire.failIfNoSpecifiedTests=false clean install

Public SDK smoke with two TableClient instances and a real OpenTelemetry collector:

Action	Exported gauge points
Create two pools with distinct pool names	10 points across four instruments
Close the first pool	Five points belonging only to the second
Close the first pool again	Gauge values remain unchanged; repeated close is delegated to the backend
Close both pools	None

NOOP registrations can be closed repeatedly. The same scenario fails when callback handles are discarded, reproducing the original leak.

The native OpenTelemetry backend logs a warning for repeated close; the wrapper adds no atomics, mutable state or duplicate-close guard.

CI compatibility

The previous JDBC BadCustomMeter compilation failure is fixed by retaining the old void signature. JDBC sources are unchanged. Local database integration tests were not run because Docker is not running; the CI matrix will exercise the complete downstream suites.

Deprecation warnings from compatibility tests and native OpenTelemetry warnings on repeated close are expected and not suppressed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.69%. Comparing base (79ef25c) to head (bc34550).

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #735      +/-   ##
============================================
+ Coverage     74.54%   74.69%   +0.14%     
- Complexity     3654     3667      +13     
============================================
  Files           393      394       +1     
  Lines         16606    16623      +17     
  Branches       1756     1756              
============================================
+ Hits          12379    12416      +37     
+ Misses         3603     3593      -10     
+ Partials        624      614      -10     

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