feat(ddtrace/tracer): transmit knuth sampling rate to backend

[darccio]

What does this PR do?

Attempts to implement propagation for _dd.p.ksr.

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!

[darccio]

@codex review

[pr-commenter[bot]]

Benchmarks

Benchmark execution time: 2025-12-01 18:50:41

Comparing candidate commit ac9f6aebcbbd5b6275e8314ad87e6a716f1fac54 in PR branch dario.castane/ai-devx-workshop/knuth-rfc with baseline commit cad41dc6aa7a73a358ec8681effdfc96beda83f1 in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 18 metrics, 6 unstable metrics.

[chatgpt-codex-connector[bot]]

Codex Review: Didn't find any major issues. Delightful!

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[datadog-datadog-prod-us1[bot]]

✅ 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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[darccio]

Failing tests are flaky and unrelated to the change.

[darccio]

Closing until getting more feedback from @yuanyuanzhao3