Add Live Debugger runtime benchmark coverage

[watson]

What and why?

This adds repeatable browser runtime benchmark coverage for dormant Live Debugger instrumentation. The goal is to give Live Debugger browser changes a controlled way to track whether instrumentation changes introduce measurable overhead in real browsers.

How?

Adds a Playwright benchmark under packages/tests/src/bench/liveDebuggerRuntime with Tiny and Hot workload shapes, baseline/control/instrumented variants, and SDK-like dormant probe hooks installed in the page.

Adds a custom reporter that computes conservative per-call overhead bounds, 95% confidence intervals, A/A diagnostics, moving-block bootstrap intervals, autocorrelation diagnostics, browser failures, raw JSON output, and a PR comment body.

Adds yarn workspace @dd/tests bench:live-debugger:runtime, a dedicated Playwright config, result artifacts, a non-blocking CI job, a shared Playwright setup action, and Live Debugger contributor docs explaining how to run and interpret the benchmark.

For more details, see the "Runtime benchmark" section in the added CONTRIBUTING.md file in this PR.

Validation

The benchmark runs in CI as a non-blocking job and uploads raw samples plus the generated report artifacts for review.

Benchmark note

The current benchmark numbers show a significant dormant-instrumentation overhead in Safari. Most of this is fixed in stacked PR #438.

[watson]

• Add Live Debugger runtime benchmark coverage #429 : 2 dependent PRs (#438 , #440 ) 👈 (View in Graphite)
• master

This stack of pull requests is managed by Graphite. Learn more about stacking.

[github-actions]

Live Debugger Runtime Benchmark

SDK-loaded dormant-probe runtime overhead, measured against an uninstrumented bundle in the same browser session.

Browser	Workload	Quality	Per-call overhead upper
chrome	Hot	clean	<= 3.38 ns
chrome	Tiny	clean	<= 0.28 ns
firefox	Hot	clean	<= 7.70 ns
firefox	Tiny	clean	<= 0.27 ns
safari	Hot	clean	<= 13.85 ns
safari	Tiny	clean	<= 12.99 ns

Full diagnostics

browser  workload  quality  per-call overhead upper  overhead upper           95% CI        A/A diag         block CI  acf(1)   baseline  instrumented                         samples
-------  --------  -------  -----------------------  --------------  ---------------  --------------  ---------------  ------  ---------  ------------  ------------------------------
chrome   Hot       clean                 <= 3.38 ns        <= 7.82%    3.37..3.38 ns  -0.01..0.01 ns    3.37..3.38 ns   -0.06  51.250 ms     55.255 ms  102 (trim 20%, outliers 11.8%)
chrome   Tiny      clean                 <= 0.28 ns        <= 4.40%    0.23..0.28 ns  -0.01..0.06 ns    0.20..0.28 ns    0.06  53.010 ms     55.140 ms  102 (trim 20%, outliers 18.6%)
firefox  Hot       clean                 <= 7.70 ns       <= 36.15%    7.68..7.70 ns  -0.00..0.01 ns    7.68..7.69 ns    0.02  40.660 ms     55.360 ms  102 (trim 20%, outliers 15.7%)
firefox  Tiny      clean                 <= 0.27 ns        <= 2.27%    0.27..0.27 ns  -0.00..0.00 ns    0.27..0.27 ns    0.08  58.940 ms     60.280 ms   102 (trim 20%, outliers 7.8%)
safari   Hot       clean                <= 13.85 ns       <= 65.86%  13.63..13.80 ns  -0.00..0.00 ns  13.59..13.85 ns    0.02  32.980 ms     54.520 ms   102 (trim 20%, outliers 2.0%)
safari   Tiny      clean                <= 12.99 ns      <= 396.82%  12.74..12.92 ns  -0.00..0.00 ns  12.65..12.99 ns    0.23  12.000 ms     59.170 ms   102 (trim 20%, outliers 2.9%)

Raw samples are in the live-debugger-runtime-bench-results artifact.

[datadog-datadog-prod-us1-2]

✅ All CI checks and tests passed.

🎉 All green!

🧪 All tests passed
❄️ No new flaky tests detected

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

LGTM, but I'll defer the CI changes and test configuration to the build-plugins team