Add W3C Trace Context propagation via MCP _meta

[ChrisJBurns]

Summary

• Add MetaCarrier implementing propagation.TextMapCarrier for MCP _meta fields
• Add InjectMetaTraceContext for injecting traceparent/tracestate into outgoing MCP requests
• Enable distributed tracing across vMCP → backend boundaries using standard W3C Trace Context format

This is the foundation layer for the MCP OTEL semantic conventions alignment (#3399). The propagation carrier and injection function are used by subsequent PRs to inject trace context into outgoing MCP calls.

PR Stack (1/3): This PR → #3683 (metrics/attributes) → #3 (client spans)

Test plan

• Unit tests for MetaCarrier (Get/Set/Keys/Meta)
• Unit tests for InjectMetaTraceContext (with real tracer, nil meta)
• Tests that mutate global OTEL propagator are non-parallel (with //nolint directive)
• go build ./... passes
• go test ./pkg/telemetry/... passes
• task lint-fix passes with 0 issues

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 66.32%. Comparing base (2c00fae) to head (903fb98).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3682   +/-   ##
=======================================
  Coverage   66.31%   66.32%           
=======================================
  Files         426      427    +1     
  Lines       41742    41765   +23     
=======================================
+ Hits        27682    27700   +18     
- Misses      11948    11955    +7     
+ Partials     2112     2110    -2     

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[jhrozek]

Review conducted with 4 specialized agents (MCP Protocol Expert, Go Expert Developer, Security Reviewer, Code Reviewer). 2 inline comments below, no blocking issues found.

Overall: Clean, correct, well-tested implementation. MCP spec-compliant use of _meta for W3C Trace Context. No security concerns.

[jhrozek]

@claude could you rebase the commit atop origin/main and see if you can address my review comments in a separate commit (they are squashed anyway and a separate commit will be easier to back away from)

[claude]

Claude finished @jhrozek's task in 2m 24s —— View job

---

Review comments addressed ✓

• Fetch latest from origin/main and rebase (already based on recent commit)
• Read current test file to understand structure
• Read propagation.go file
• Refactor test to table-driven format
• Add compile-time assertion for TextMapCarrier interface
• Run tests to verify changes (all tests pass)
• Commit changes as separate commit
• Push to remote branch

Changes Made

I've successfully addressed both review comments in a separate commit (125f2d1):

1. Added compile-time assertion (pkg/telemetry/propagation.go):

   // Compile-time assertion that MetaCarrier implements propagation.TextMapCarrier
   var _ propagation.TextMapCarrier = (*MetaCarrier)(nil)

   This ensures at compile time that MetaCarrier correctly implements the propagation.TextMapCarrier interface.

2. Refactored to table-driven test (pkg/telemetry/propagation_test.go):

   • Converted the Get operations in TestMetaCarrier_GetSetKeys to use a table-driven test pattern with subtests
   • This makes the test more compact and easier to extend with additional test cases

All tests pass successfully. The linter check will run in CI since it requires tool approval.

--- | Branch: otel/propagation-foundation

[jhrozek]

welp, I don't know if the table-driven test is an improvement

[ChrisJBurns]

@claude Address the lint errors that occur in the pipeline please

[claude]

Claude finished @ChrisJBurns's task in 1m 13s —— View job

---

Lint errors fixed ✓

• Check CI status to identify lint errors
• Fix paralleltest/tparallel issues in pkg/telemetry/propagation_test.go
• Commit and push changes

What was fixed:

• Added t.Parallel() call to subtests in the table-driven test in TestMetaCarrier_GetSetKeys
• This satisfies both the paralleltest and tparallel linters

Changes committed:

• Commit: 9ec55e4
• The lint errors should now be resolved in the CI pipeline

--- | | Branch: otel/propagation-foundation