[W3C] Refactor W3C integration tests

[martincostello]

Changes

Improve developer experience when troubleshooting failing tests (like I was in #6307) by logging both stderr and stdout and ignoring warnings in the upstream Python tests.

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 86.74%. Comparing base (dde8f40) to head (e6fc093).
Report is 6 commits behind head on main.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6328      +/-   ##
==========================================
- Coverage   86.76%   86.74%   -0.02%     
==========================================
  Files         258      258              
  Lines       11879    11879              
==========================================
- Hits        10307    10305       -2     
- Misses       1572     1574       +2     

Flag	Coverage Δ
unittests-Project-Experimental	86.66% <ø> (+0.10%)	⬆️
unittests-Project-Stable	86.57% <ø> (ø)

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

see 2 files with indirect coverage changes

[Copilot]

Pull Request Overview

Refactors W3C integration tests to capture both stdout and stderr and ignore Python warnings, improving failure diagnostics.

• Passes the -W ignore flag to suppress upstream warnings in Python tests.
• Changes RunCommand to return both stdout and stderr.
• Updates test logic to log and parse the new streams.

Comments suppressed due to low confidence (2)

test/OpenTelemetry.Instrumentation.W3cTraceContext.Tests/W3CTraceContextTests.cs:89

• [nitpick] The new tuple return type would benefit from an XML doc comment summarizing the meaning of StdOut and StdErr, improving maintainability and discoverability.

private static (string StdOut, string StdErr) RunCommand(string command, string args)

test/OpenTelemetry.Instrumentation.W3cTraceContext.Tests/W3CTraceContextTests.cs:71

• [nitpick] Consider adding a unit test or integration test for RunCommand to verify correct separation of stdout and stderr and the fallback parsing behavior.

(var stdout, var stderr) = RunCommand("python", "-W ignore trace-context/test/test.py http://localhost:5000/");

[Copilot]

Sequential calls to ReadToEnd on both StandardOutput and StandardError can lead to deadlocks for large outputs; consider using asynchronous reading (e.g., BeginOutputReadLine) or reading both streams in parallel.

[martincostello]

I have some code of my own that does this I can copy over here if a human reviewer wants me to.

[rajkumar-rangaraj]

Yes, it's better to cover it. Later, if we encounter such issues, it will be very difficult to troubleshoot and determine if the issue is due to this reason.