RUM-17106: Test Only Flush Client-Side-Stats before shared write executor shuts down

[abrooksv]

What does this PR do?

Add StatsConcentrator.drainAndFlush() and wire it into flushAndShutdownExecutors() via a new "flush_and_stop_stats" event, so CSS buckets are synchronously flushed before the shared write executor is shut down instead of being dropped when the periodic timer hasn't fired yet.

This follows the pattern that RUM Monitor established

Motivation

This is used by RUM Fit to flush the CSS data during test runs

[codecov-commenter]

Codecov Report

❌ Patch coverage is 91.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.33%. Comparing base (8d12b19) to head (7bbce05).

Files with missing lines	Patch %	Lines
...tadog/android/trace/internal/ClientStatsFeature.kt	92.31%	0 Missing and 1 partial ⚠️
...trace/internal/domain/metrics/StatsConcentrator.kt	85.71%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@                      Coverage Diff                      @@
##           feature/client-side-stats    #3603      +/-   ##
=============================================================
+ Coverage                      73.29%   73.33%   +0.04%     
=============================================================
  Files                            986      986              
  Lines                          35988    36011      +23     
  Branches                        6066     6070       +4     
=============================================================
+ Hits                           26375    26406      +31     
+ Misses                          7930     7922       -8     
  Partials                        1683     1683              

Files with missing lines	Coverage Δ
...ore/src/main/kotlin/com/datadog/android/Datadog.kt	80.00% <100.00%> (+6.09%)	⬆️
...tadog/android/trace/internal/ClientStatsFeature.kt	90.70% <92.31%> (+0.70%)	⬆️
...trace/internal/domain/metrics/StatsConcentrator.kt	96.20% <85.71%> (-0.41%)	⬇️

... and 40 files with indirect coverage changes

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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Drain context work before stopping stats aggregation

When spans were just published, StatsConcentrator.record() may still have only enqueued its withContext callback on the core context executor; this sends flush_and_stop_stats before (sdkCore as? DatadogCore)?.flushStoredData() drains that executor. In that case drainAndFlush() shuts down the stats executor, and the later-drained context callback rejects stats-aggregate, so the newest client-side stats are dropped during the test flush this change is meant to make reliable.

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[chatgpt-codex-connector]

Avoid waiting on the periodic stats flush during drain

In a normally initialized client-stats feature there is always a 30-second delayed schedulePeriodicFlush() task in this ScheduledThreadPoolExecutor; queue.drainTo does not remove unexpired delayed tasks, and shutdown() leaves them queued, so this await commonly blocks for the full 10-second timeout before the final synchronous flush. Calls to the internal test flush soon after initialization will therefore get an avoidable 10-second stall unless the delayed task is cancelled/removed or shutdownNow()/shutdown policy is used before awaiting.

Useful? React with 👍 / 👎.