stats/opentelemetry: add e2e test for baggage propagation

[ulascansenturk]

Summary

Fixes #8520.

OpenTelemetry baggage already propagates correctly through the gRPC pipeline when the W3C Baggage propagator is configured — the tracing carriers (IncomingCarrier/OutgoingCarrier) are generic TextMapCarriers used by the configured propagator's Inject/Extract, so baggage rides in the baggage metadata header exactly like trace context. No production code change is needed; the gap was the lack of a test proving it.

This PR adds TestRPC_BaggagePropagation, which:

• Sets baggage (key1=value1, key2=value2) on the client context.
• Configures propagation.Baggage{} in the trace options.
• Asserts the server handler observes the same baggage, for both a unary and a streaming RPC.

Verification

• Passes with -race.
• Confirmed meaningful: removing propagation.Baggage{} makes the test fail (server sees empty baggage), so it exercises propagation rather than passing vacuously.
• go vet and gofmt clean.

RELEASE NOTES: none

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.17%. Comparing base (adc97de) to head (8e96a8f).
⚠️ Report is 22 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9160      +/-   ##
==========================================
- Coverage   83.20%   83.17%   -0.03%     
==========================================
  Files         418      419       +1     
  Lines       33709    33824     +115     
==========================================
+ Hits        28048    28134      +86     
- Misses       4245     4264      +19     
- Partials     1416     1426      +10     

see 37 files with indirect coverage changes

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[mbissa]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces a new end-to-end test, TestRPC_BaggagePropagation, to verify that OpenTelemetry baggage is correctly propagated from the client to the server across both unary and streaming RPCs. Feedback on the changes points out a critical issue in the streaming RPC handler (FullDuplexCallF), where a non-EOF error returned by stream.Recv() would result in an infinite loop and high CPU usage. It is recommended to update the loop to handle and return any non-nil errors.

[gemini-code-assist]

If stream.Recv() returns an error other than io.EOF (such as a canceled context or a transport error), the loop will continue infinitely because there is no exit condition for other errors. This can cause a tight loop spinning at 100% CPU. We should return the error if it is not nil.

			for {
				if _, err := stream.Recv(); err != nil {
					if err == io.EOF {
						return nil
					}
					return err
				}
			}

[ulascansenturk]

Good catch. Fixed: the loop now returns the error when it isn't io.EOF, so a canceled context or transport error no longer spins at 100% CPU.

[mbissa]

Thanks @ulascansenturk for this contribution. Assigning to @easwars for second set of eyes.