dd-trace-go chore: hold RLock when reading meta/metric maps

What does this PR do?

Ensures that span.metric and span.meta are read while holding span.mu.RLock. Targets areas where the lock is not already held and we are not in a StartSpan function.

The main issue was that, when finishing a span, we only want to set trace tags on the first finished span. In some cases, that is the same span that is passed to tracer.finish(). This span is already locked. However, in other cases, we get the first finished span from finishedSpans[0]. This span is not already locked. This PR adds a lock around this span when setting its metric and meta tags.

Motivation

#incident-46344

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.

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Dec 11 '25 19:12 hannahkm

Benchmarks

Benchmark execution time: 2025-12-17 13:33:03

Comparing candidate commit 07c776eaedef1cd5616741eaf4fbffd3940c797e in PR branch hannahkm/rlock-meta with baseline commit c8e44a3d84ad2e60fce8394dcd24b3ba01ff58b8 in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 15 metrics, 0 unstable metrics.

Dec 11 '25 19:12 pr-commenter[bot]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 54.37%. Comparing base (bb69888) to head (7fc6441). :warning: Report is 1 commits behind head on main.

Additional details and impacted files

Files with missing lines	Coverage Δ
ddtrace/tracer/spancontext.go	`90.18% <100.00%> (+0.12%)`	:arrow_up:

... and 7 files with indirect coverage changes

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Dec 11 '25 19:12 codecov[bot]

✅ Tests

🎉 All green!

❄️ No new flaky tests detected
🧪 All tests passed

_{This comment will be updated automatically if new data arrives.

🔗 Commit SHA: 07c776e | Docs | Datadog PR Page | Was this helpful? Give us feedback!}

Dec 11 '25 19:12 datadog-datadog-prod-us1[bot]

/merge

Dec 15 '25 15:12 hannahkm

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2025-12-15 15:21:00 UTC :information_source: Start processing command /merge

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2025-12-15 19:22:10 UTC :warning: MergeQueue: This merge request was unqueued

devflow unqueued this merge request: It did not become mergeable within the expected time

Dec 15 '25 15:12 dd-devflow-routing-codex[bot]

/merge

Dec 16 '25 10:12 darccio

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2025-12-16 10:13:59 UTC :information_source: Start processing command /merge

2025-12-16 10:14:09 UTC :information_source: MergeQueue: waiting for PR to be ready

This pull request is not mergeable according to GitHub. Common reasons include pending required checks, missing approvals, or merge conflicts — but it could also be blocked by other repository rules or settings. It will be added to the queue as soon as checks pass and/or get approvals. Note: if you pushed new commits since the last approval, you may need additional approval. You can remove it from the waiting list with /remove command.

2025-12-16 14:15:15 UTC :warning: MergeQueue: This merge request was unqueued

devflow unqueued this merge request: It did not become mergeable within the expected time

Dec 16 '25 10:12 dd-devflow-routing-codex[bot]

@codex review

Dec 16 '25 15:12 darccio

Codex Review: Didn't find any major issues. Breezy!

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Dec 16 '25 15:12 chatgpt-codex-connector[bot]

/merge

Dec 17 '25 13:12 darccio

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2025-12-17 13:23:37 UTC :information_source: Start processing command /merge

2025-12-17 13:23:46 UTC :information_source: MergeQueue: waiting for PR to be ready

This pull request is not mergeable according to GitHub. Common reasons include pending required checks, missing approvals, or merge conflicts — but it could also be blocked by other repository rules or settings. It will be added to the queue as soon as checks pass and/or get approvals. Note: if you pushed new commits since the last approval, you may need additional approval. You can remove it from the waiting list with /remove command.

2025-12-17 13:46:16 UTC :information_source: MergeQueue: merge request added to the queue

The expected merge time in main is approximately 44m (p90).

2025-12-17 14:13:52 UTC :information_source: MergeQueue: This merge request was merged

Dec 17 '25 13:12 dd-devflow-routing-codex[bot]