ddtrace/tracer: remove agent_psr when rule_psr is set

[dianashevchenko]

What does this PR do?

APMAPI-22
Removes the '_dd.agent_psr' metric when '_dd.rule_psr' is set through sampling rules.

Motivation

The backend currently handles this because there is a precedence rule, but it should be fixed. Impact is sampling “reason” shown to the user in the frontend can be misleading/wrong

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!

[pr-commenter]

Benchmarks

Benchmark execution time: 2024-04-30 12:59:26

Comparing candidate commit 6fa2b0a in PR branch shevchenko/remove-agent_psr-when-rule-sampling with baseline commit 155ef2d in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 43 metrics, 1 unstable metrics.

[darccio]

LGTM