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

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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 client info and 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!

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Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• feat(contrib/mcp-go): Record MCP Session ID as tag #4106
• feat(contrib/mcp-go): Trace MCP session initializations with MLObs spans #4101 👈 (View in Graphite)
• feat(contrib/mcp-go): Initial mcp-go tracer implementation #4100
• main

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[pr-commenter]

Benchmarks

Benchmark execution time: 2025-11-05 21:38:58

Comparing candidate commit 498ae3a in PR branch jb/contrib-mcp-go-init-span with baseline commit b00ba8a in branch jb/contrib-mcp-go-base.

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