ddtrace/tracer: fixed resampling to occur on root span only

[dianashevchenko]

What does this PR do?

This PR updates sampling logic. Now, resampling only occurs on root spans. Neither"_dd.limit_psr", nor "_dd.rule_psr" should be present on the child span.
Locking the span after context propagation prevents re-sampling.

Motivation

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.
• 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!

[darccio]

LGTM

[pr-commenter]

Benchmarks

Benchmark execution time: 2024-08-21 13:49:30

Comparing candidate commit ed09cd1 in PR branch shevchenko/sample-tags-root-only with baseline commit e4985df in branch main.

Found 0 performance improvements and 1 performance regressions! Performance is the same for 57 metrics, 1 unstable metrics.

scenario:BenchmarkSetTagString-24

• 🟥 execution_time [+6.192ns; +8.108ns] or [+5.489%; +7.187%]