ddtrace/tracer: fix sampling rule override for manual keep

[dianashevchenko]

What does this PR do?

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.

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[pr-commenter]

Benchmarks

Benchmark execution time: 2024-04-19 13:32:45

Comparing candidate commit fa9237d in PR branch shevchenko/manual-keep-bug with baseline commit fda4cc5 in branch main.

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