ddtrace/tracer: added support for metrics tag sampling

[dianashevchenko]

What does this PR do?

This PR extends the support for sampling rules. Previously, sampling was ran on meta tags only, now it is available for the numerics as well.

Also, this PR fixes case sensitivity of the string tags (for both keys and values)

Motivation

The Extended Sampling RFC

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: 2024-04-18 18:39:39

Comparing candidate commit 5b3abba in PR branch shevchenko/sampling-by-metric-tag with baseline commit af0486a in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 44 metrics, 0 unstable metrics.