otel sampling fix

[mtoffl01]

What does this PR do?

Locks the trace sampling priority when it's been inherited by a parent span context.

Motivation

APMS-15887

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.

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[pr-commenter]

Benchmarks

Benchmark execution time: 2025-07-02 17:54:16

Comparing candidate commit 21ca316 in PR branch mtoff/otel-sampling-fix with baseline commit f94844a in branch v1-maintenance.

Found 1 performance improvements and 0 performance regressions! Performance is the same for 60 metrics, 1 unstable metrics.

scenario:BenchmarkSetTagStringer-24

• 🟩 execution_time [-7.816ns; -3.924ns] or [-5.277%; -2.650%]

[darccio]

@mtoffl01 This causes a deadlock because t is already locked at the beginning of finishedOne.

[mtoffl01]

Closing in favor of #3718