[OpenTelemetry] Fix SemaphoreSlim leak

[martincostello]

Changes

Fix SemaphoreSlim.WaitAsync() waiter leak.

Merge requirement checklist

• CONTRIBUTING guidelines followed (license requirements, nullable enabled, static analysis, etc.)
• Unit tests added/updated
• Appropriate CHANGELOG.md files updated for non-trivial changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.61%. Comparing base (a2b6372) to head (f0b6a9e).
⚠️ Report is 3 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7069      +/-   ##
==========================================
+ Coverage   88.54%   88.61%   +0.07%     
==========================================
  Files         270      270              
  Lines       12884    12938      +54     
==========================================
+ Hits        11408    11465      +57     
+ Misses       1476     1473       -3     

Flag	Coverage Δ
unittests-Project-Experimental	88.60% <100.00%> (+0.14%)	⬆️
unittests-Project-Stable	88.20% <100.00%> (-0.27%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...rc/OpenTelemetry/Internal/BatchExportTaskWorker.cs	44.66% <100.00%> (+6.79%)	⬆️
...nternal/PeriodicExportingMetricReaderTaskWorker.cs	75.00% <100.00%> (+9.32%)	⬆️

... and 4 files with indirect coverage changes

[Copilot]

Pull request overview

Fixes a SemaphoreSlim.WaitAsync() waiter leak in task-based workers used when threading is disabled (e.g., Blazor/WASM), ensuring TriggerExport() can reliably wake the worker loop without accumulating orphaned waiters.

Changes:

• Cancel both the delay and exportTrigger.WaitAsync(...) using a linked CancellationTokenSource to prevent SemaphoreSlim waiter leaks in task-based workers.
• Add new unit tests validating that trigger-based exports/collects don’t wait for the scheduled delay/export interval after the worker has been idle.
• Update src/OpenTelemetry/CHANGELOG.md with a note about the resource leak fix.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

File	Description
test/OpenTelemetry.Tests/Internal/TaskWorkerTests.cs	Adds regression tests for trigger responsiveness after idle cycles in task-based workers.
src/OpenTelemetry/Internal/PeriodicExportingMetricReaderTaskWorker.cs	Cancels WaitAsync waiters when the delay wins, avoiding leaked semaphore waiters.
src/OpenTelemetry/Internal/BatchExportTaskWorker.cs	Cancels WaitAsync waiters when the delay wins, avoiding leaked semaphore waiters.
src/OpenTelemetry/CHANGELOG.md	Documents the resource leak fix.

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