chore(config): migrate tracer.runtimeMetrics and tracer.runtimeMetricsV2

[mtoffl01]

What does this PR do?

Tracer migrates to using Config's runtimeMetrics and runtimeMetricsV2 instead of its own config.runtimeMetrics and config.runtimeMetricsV2.

Motivation

datadoghq.atlassian.net/browse/APMAPI-1748

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: 2025-12-09 16:37:46

Comparing candidate commit a437604 in PR branch mtoff/runtime-metrics with baseline commit 78b2dc1 in branch main.

Found 0 performance improvements and 1 performance regressions! Performance is the same for 17 metrics, 6 unstable metrics.

scenario:BenchmarkParallelMetrics/distribution/handle-reused-25

• 🟥 execution_time [+1.393ns; +7.349ns] or [+2.147%; +11.330%]

[datadog-official]

✅ Tests

🎉 All green!

❄️ No new flaky tests detected
🧪 All tests passed

_{This comment will be updated automatically if new data arrives.
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