ddtrace/tracer: update sample rate configuration parsing to avoid logging a warning when empty

[zacharycmontoya]

What does this PR do?

This PR adds a small rewrite to the parsing of the DD / OTEL Sample Rate configuration so that we do not log a warning when the sample rate is empty. This also adds a regression test for the scenario.

Motivation

Fixes #2749

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: 2024-06-18 17:51:46

Comparing candidate commit d4ff6c5 in PR branch zach.montoya/log-dd-trace-sample-rate with baseline commit 8679af2 in branch main.

Some scenarios are present only in baseline or only in candidate runs. If you didn't create or remove some scenarios in your branch, this maybe a sign of crashed benchmarks 💥💥💥
Check Gitlab CI job log to find if any benchmark has crashed.

Scenarios present only in candidate:

• BenchmarkInjectW3C-24

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