Fix flaky MicrometerTest (ConditionTimeoutException)

[xiang17]

This PR is trying to fix the frequent MicrometerTest is flaky failures.

MicrometerTest waited on AzureMonitorMeterRegistry's background publisher. Micrometer's StepLong/StepDouble only expose a recorded value if it is polled in the very next step (previous = (lastInit == stepTime - 1) ? v : 0), so a single delayed publish tick on a busy CI runner discards the recorded values permanently and the await() can never succeed. Raising the awaitility timeout to 30 seconds (#4746) did not help for that reason - the data is lost, not late; it recently failed again in both shouldCaptureTimer and shouldCaptureDistributionSummary.

Tests now drive the registry with a MockClock and publish explicitly, so results are deterministic and failures are ordinary assertion errors that show expected vs. actual instead of an opaque timeout.

Current failure examples

> Task :agent:instrumentation:micrometer-1.0:test

MicrometerTest > shouldCaptureTimer() FAILED
    org.awaitility.core.ConditionTimeoutException: Condition with Lambda expression in MicrometerTest was not fulfilled within 30 seconds.
        at app//org.awaitility.core.ConditionAwaiter.await(ConditionAwaiter.java:167)
        at app//org.awaitility.core.CallableCondition.await(CallableCondition.java:78)
        at app//org.awaitility.core.CallableCondition.await(CallableCondition.java:26)
        at app//org.awaitility.core.ConditionFactory.until(ConditionFactory.java:1160)
        at app//org.awaitility.core.ConditionFactory.until(ConditionFactory.java:1129)
        at app//MicrometerTest.shouldCaptureTimer(MicrometerTest.java:135)

> Task :agent:instrumentation:micrometer-1.0:test

MicrometerTest > shouldCaptureDistributionSummary() FAILED
    org.awaitility.core.ConditionTimeoutException: Condition with Lambda expression in MicrometerTest was not fulfilled within 30 seconds.
        at app//org.awaitility.core.ConditionAwaiter.await(ConditionAwaiter.java:167)
        at app//org.awaitility.core.CallableCondition.await(CallableCondition.java:78)
        at app//org.awaitility.core.CallableCondition.await(CallableCondition.java:26)
        at app//org.awaitility.core.ConditionFactory.until(ConditionFactory.java:1160)
        at app//org.awaitility.core.ConditionFactory.until(ConditionFactory.java:1129)
        at app//MicrometerTest.shouldCaptureDistributionSummary(MicrometerTest.java:158)

What a failure would look like after this PR

MicrometerTest > shouldCaptureTimer() FAILED
    java.lang.AssertionError: [non-zero measurements named "test-timer-DEMO", out of everything published: [test-timer=99.0]]
    Expecting actual not to be empty
        at MicrometerTest.getLastMeasurement(MicrometerTest.java:241)
        at MicrometerTest.shouldCaptureTimer(MicrometerTest.java:124)

For significant contributions please make sure you have completed the following items:

• Design discussion issue #
• Changes in public surface reviewed
• CHANGELOG.md updated