feat(tracer): add health metrics configuration support

[mtoffl01]

What does this PR do?

Introduces env var DD_HEALTH_METRICS_ENABLED and programmatic API WithHealthMetrics to disable health metrics, which report by default.

Motivation

This feature could potentially be noisy and/or introduce performance regressions. Users should be able to disable functionality if need be.
Note: the name of the env var was pulled from the feature parity dashboard

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.
• New code is free of linting errors. You can check this by running ./scripts/lint.sh locally.
• 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: 2025-09-22 20:28:34

Comparing candidate commit 61224fe in PR branch mtoff/disable-health-metrics with baseline commit 3119b73 in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 24 metrics, 0 unstable metrics.

[hannahkm]

lgtm!!

[hannahkm]

Failures look like flakes + API changes. I didn't realize this was still in draft, but once those failures get fixed it looks good to go from me 😅