fix(mcp): propagate trace context when metadata is absent

[patrickswedish]

Creates MCP request metadata when callers omit it, injects the full W3C trace carrier while preserving existing metadata, and adds regression coverage for absent and pre-populated metadata. Verification: all 7 MCP instrumentation tests pass; Ruff passes on both changed files. Fixes #4374.

Summary by CodeRabbit

• Bug Fixes
  • Improved trace-context propagation for MCP client requests when request metadata is missing.
  • Preserves any existing custom request metadata while adding trace context.
  • Injects the full set of available trace-context fields (e.g., traceparent and tracestate) into request metadata.
• Tests
  • Added unit tests for both missing-metadata and pre-existing-custom-metadata scenarios to verify correct injection behavior.

[CLAassistant]

All committers have signed the CLA.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: ba562de5-dea9-4067-9a1a-53814bfe5297

📥 Commits

Reviewing files that changed from the base of the PR and between 0367276 and 5773bd8.

📒 Files selected for processing (1)

• packages/opentelemetry-instrumentation-mcp/tests/test_client_trace_context.py

🚧 Files skipped from review as they are similar to previous changes (1)

• packages/opentelemetry-instrumentation-mcp/tests/test_client_trace_context.py

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📝 Walkthrough

Walkthrough

The MCP client instrumentation now injects all propagated trace-context fields into request metadata, creates metadata when absent, preserves existing custom metadata, and adds parametrized tests for both cases.

Changes

MCP trace context propagation

Layer / File(s)	Summary
Metadata injection and validation packages/opentelemetry-instrumentation-mcp/opentelemetry/instrumentation/mcp/instrumentation.py, packages/opentelemetry-instrumentation-mcp/tests/test_client_trace_context.py	patch_mcp_client creates RequestParams.Meta() when needed, injects the full carrier into params.meta, and preserves existing custom metadata; tests cover absent and pre-existing metadata.

Estimated code review effort: 2 (Simple) | ~10 minutes

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly summarizes the main fix: propagating trace context when MCP metadata is absent.
Linked Issues check	✅ Passed	The changes create metadata when absent, inject full trace context, preserve existing fields, and add regression tests as requested.
Out of Scope Changes check	✅ Passed	The PR stays focused on MCP trace-context propagation and related tests, with no obvious unrelated code changes.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In
`@packages/opentelemetry-instrumentation-mcp/tests/test_client_trace_context.py`:
- Around line 30-41: Update the regression test around patch_mcp_client to
exercise the tools/call branch: set message.root.method to "tools/call",
configure the instrumentor’s _handle_tool_call mock to return "sent", and assert
that result while preserving the existing trace metadata assertions.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: ce3eddf0-c7e6-4a38-8a5d-f65f3cab0c5a

📥 Commits

Reviewing files that changed from the base of the PR and between 93429cf and fda8f7e.

📒 Files selected for processing (2)

• packages/opentelemetry-instrumentation-mcp/opentelemetry/instrumentation/mcp/instrumentation.py
• packages/opentelemetry-instrumentation-mcp/tests/test_client_trace_context.py