fix(ddtrace/tracer): TestTracerTwiceStartRuntimeMetrics

[felixge]

What does this PR do?

Calling tracer.Start() implicitly stops any existing tracer before starting a new one. This was not working correctly for runtime metrics v2, causing a second Start() call to stop the existing runtime metrics v2 emitter, but not starting a new one and printing an error to the log instead.

This PR adds a test case to reproduce the issue as well as a fix.

Motivation

ZD 2321693 has an example of somebody running into this issue.

It's possible that the problem in that ticket is caused by an unintended second call to Start(), but that's a separate issue.

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!

[felixge]

/merge

[dd-devflow-routing-codex]

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2025-10-30 14:52:41 UTC ℹ️ Start processing command /merge

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2025-10-30 18:53:07 UTC ⚠️ MergeQueue: This merge request was unqueued

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