feat(contrib/mcp-go): Trace MCP session initializations with MLObs spans

[jboolean]

NB: This is part of a stack

What does this PR do?

In addition to tool calls, record task spans for session initialization.

Motivation

We want to record data like clientInfo during session Initialization and this seems like the best way to do it.

I considered a metric, but in the future we will also want to tag the session ID, and metrics do not do well with unique tags. Also, this allows viewing everything together in the LLMObs UI.

A writeup with more context on this design

Relates to MLOB-4373

Future

In future PRs I will add tags for session ID.

Reviewer's Checklist

• Changed code has unit tests for its functionality at or near 100% coverage.
• System-Tests covering this feature have been added and enabled with the va.b.c-dev version tag.
• There is a benchmark for any new code, or changes to existing code.
• If this interacts with the agent in a new way, a system test has been added.
• New code is free of linting errors. You can check this by running ./scripts/lint.sh locally.
• Add an appropriate team label so this PR gets put in the right place for the release notes.
• Non-trivial go.mod changes, e.g. adding new modules, are reviewed by @DataDog/dd-trace-go-guild.

Unsure? Have a question? Request a review!

[jboolean]

• feat(contrib/mcp-go): Trace MCP session initializations with MLObs spans #4101 : 3 dependent PRs (#4106 , #4115 , #4122 ) 👈 (View in Graphite)
• feat(contrib/mcp-go): Initial mcp-go tracer implementation #4100
• main

This stack of pull requests is managed by Graphite. Learn more about stacking.

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-11-14 15:39:44

Comparing candidate commit 7f50759 in PR branch jb/contrib-mcp-go-init-span with baseline commit 4be1146 in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 24 metrics, 0 unstable metrics.

[datadog-official]

✅ Tests

🎉 All green!

❄️ No new flaky tests detected
🧪 All tests passed

_{This comment will be updated automatically if new data arrives.
🔗 Commit SHA: 7f50759 | Docs | Datadog PR Page | Was this helpful? Give us feedback!}

[jboolean]

I made an update that removes the ttlcache. This was a relic of long term session tracking. I read the mcp-go code and did not see any code paths that would lead to the after or error hooks not being called, so records will always be deleted. This simplifies the design by removing the cleanup function.

It would be hard to support via orchestrion with the cleanup.

[jboolean]

@genesor Could you take a look please?

[jboolean]

@hannahkm Would love your review so I can get this in and hopefully soon integrated with dd-source. Thanks!

[jboolean]

/merge

[dd-devflow-routing-codex]

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2025-11-18 15:00:39 UTC ℹ️ Start processing command /merge

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2025-11-18 15:00:44 UTC ℹ️ MergeQueue: pull request added to the queue

The expected merge time in main is approximately 22m (p90).

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2025-11-18 15:31:25 UTC ℹ️ MergeQueue: This merge request was merged