[OpenTelemetry] Help diagnose dropped traces

[martincostello]

Fixes #7426

Changes

• Add diagnostic event when an activity is dropped due to the local parent not being sampled.
• Cover the scenario in the troubleshooting guide.

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

❌ Patch coverage is 90.90909% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 90.00%. Comparing base (2f02a19) to head (1e17e9d).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/OpenTelemetry/Trace/TracerProviderSdk.cs	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7427      +/-   ##
==========================================
- Coverage   90.01%   90.00%   -0.02%     
==========================================
  Files         277      277              
  Lines       14642    14653      +11     
==========================================
+ Hits        13180    13188       +8     
- Misses       1462     1465       +3     

Flag	Coverage Δ
unittests-Project-Experimental	89.85% <90.90%> (-0.09%)	⬇️
unittests-Project-Stable	89.79% <90.90%> (-0.19%)	⬇️

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

Files with missing lines	Coverage Δ
...nTelemetry/Internal/OpenTelemetrySdkEventSource.cs	67.01% <100.00%> (+1.45%)	⬆️
src/OpenTelemetry/Trace/TracerProviderSdk.cs	99.04% <75.00%> (-0.32%)	⬇️

... and 4 files with indirect coverage changes

[joaopgrassi]

Awesome, that would have saved me some hours (and tokens, lol). Thank you!

[Copilot]

Pull request overview

Adds self-diagnostics and documentation to help users understand why their custom spans/activities may not be exported when they have an in-process (local) parent that is not recorded, which commonly happens in ASP.NET Core scenarios without registering the framework ActivitySource.

Changes:

• Emit a new verbose OpenTelemetry-Sdk EventSource event when an activity is dropped due to an unsampled local parent.
• Add unit tests validating when the diagnostic event is (and is not) emitted.
• Document the scenario and mitigation options in the tracing troubleshooting guide.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 4 comments.

File	Description
src/OpenTelemetry/Trace/TracerProviderSdk.cs	Emits the new self-diagnostics event when sampling drops an activity.
src/OpenTelemetry/Internal/OpenTelemetrySdkEventSource.cs	Adds the new EventSource event (ID 58) and a guarded helper to emit it only for unsampled local parents.
test/OpenTelemetry.Tests/Trace/TracerProviderSdkTests.cs	Adds tests covering local-unsampled-parent emission and remote-parent non-emission.
docs/trace/customizing-the-sdk/README.md	Adds troubleshooting guidance for spans dropped due to an unsampled parent in common ASP.NET Core setups.

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[Kielek]

Shouldn't it by just?

Suggested change

	`AddSource("Microsoft.AspNetCore.Hosting")`. The request activity then becomes
	`AddSource("Microsoft.AspNetCore")`. The request activity then becomes

[martincostello]

I'm checking in the ASP.NET Core source, but it's definitely Microsoft.AspNetCore.Hosting for metrics: source

[martincostello]

You're right that for tracing it's Microsoft.AspNetCore: source

[martincostello]

Of course I should have just looked in the other repo first... code 😅