fix(ddtrace/tracer): replace RLock with Lock for NoDebugStack check to prevent race condition

[darccio]

What does this PR do?

Reduces to a single locking operation everything in Finish's NoDebugStack.

Motivation

Per @felixge's comment, there is a potential race condition in the NoDebugStack when finishing a span.

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 golangci-lint run 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!

[datadog-datadog-prod-us1]

Datadog Report

Branch report: dario.castane/single-lock-on-nodebugstack-finish
Commit report: 295b632
Test service: dd-trace-go

✅ 0 Failed, 4481 Passed, 64 Skipped, 3m 46.43s Total Time

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-04-16 13:36:12

Comparing candidate commit 8d7ee8a in PR branch dario.castane/single-lock-on-nodebugstack-finish with baseline commit f3d284b in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 55 metrics, 1 unstable metrics.