[Java SDK] Fix propagation of metrics set during onTimer processing.

[scwhittle]

Fixes #29099. Added unit test that verifies ProcessBundleHandler plumbing in addition to the existing FnApiRunner test which is at a lower level.

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[gemini-code-assist]

Summary of Changes

Hello @scwhittle, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request resolves an issue where metrics emitted during onTimer processing were not properly captured and propagated within the Beam Java SDK harness. It introduces a mechanism to correctly manage the execution state around timer processing, ensuring accurate reporting of user-defined metrics. The changes also include refactoring the PCollectionConsumerRegistry to centralize ExecutionState management and adding new unit tests to validate metric propagation for timers and bundle lifecycle methods.

Highlights

• Fix for Timer Metrics Propagation: Metrics set during onTimer processing were not being correctly propagated. The fix involves wrapping the FnDataReceiver for timers to ensure that the ExecutionState is properly activated and deactivated around the receiver.accept(timer) call, allowing metrics recorded within onTimer to be accurately attributed to the processing state.
• Refactoring PCollectionConsumerRegistry for ExecutionState: The logic for creating and managing ExecutionState for processing bundles has been extracted into a new, cached getProcessingExecutionState method within PCollectionConsumerRegistry. This method now uses a new ExecutionStateKey to ensure consistent state tracking and prevent redundant ExecutionState creation.
• New Unit Tests for ProcessBundleHandler: Comprehensive unit tests (testTimerMetrics and testStartFinishBundleMetrics) have been added to ProcessBundleHandlerTest. These tests verify that user-defined metrics (counters for onTimer, StartBundle, and FinishBundle) are correctly reported and propagated through the ProcessBundleHandler.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 56.93%. Comparing base (af748d0) to head (f7551f9).
⚠️ Report is 7 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master   #36576   +/-   ##
=========================================
  Coverage     56.93%   56.93%           
  Complexity     3393     3393           
=========================================
  Files          1222     1222           
  Lines        186815   186815           
  Branches       3544     3544           
=========================================
  Hits         106365   106365           
  Misses        77078    77078           
  Partials       3372     3372           

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