ci: declare FLAKY_BENCHMARKS_REGEX and document benchmarks CI

[igoragoli]

Description

• Declare an empty FLAKY_BENCHMARKS_REGEX in the benchmarks job, linked to Flaky Benchmarks Monitoring.
  • A scenario matching it still runs and reports, but doesn't fail the gate.
• Add .gitlab/BENCHMARKS.md with the benchmarks CI layout and how to mark a benchmark as flaky.

Motivation

APMSP-3925 Onboard dd-trace-* and nginx-datadog to benchmark stability monitoring

Additional Notes

YAML parses. Runs are unchanged since the declared value is empty.

Jira ticket: APMSP-2893

[pr-commenter]

Benchmarks

Benchmark execution time: 2026-10-01 13:38:17

Comparing candidate commit bf73099 in PR branch augusto/flaky-benchmarks-monitoring-docs with baseline commit df496e9 in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 8 metrics, 0 unstable metrics.

Explanation

This is an A/B test comparing a candidate commit's performance against that of a baseline commit. Performance changes are noted in the tables below as:

• 🟩 = significantly better candidate vs. baseline
• 🟥 = significantly worse candidate vs. baseline

We compute a confidence interval (CI) over the relative difference of means between metrics from the candidate and baseline commits, considering the baseline as the reference.

If the CI is entirely outside the configured SIGNIFICANT_IMPACT_THRESHOLD (or the deprecated UNCONFIDENCE_THRESHOLD), the change is considered significant.

Feel free to reach out to #apm-benchmarking-platform on Slack if you have any questions.

More details about the CI and significant changes

You can imagine this CI as a range of values that is likely to contain the true difference of means between the candidate and baseline commits.

CIs of the difference of means are often centered around 0%, because often changes are not that big:

---------------------------------(------|---^--------)-------------------------------->
                              -0.6%    0%  0.3%     +1.2%
                                 |          |        |
         lower bound of the CI --'          |        |
sample mean (center of the CI) -------------'        |
         upper bound of the CI ----------------------'

As described above, a change is considered significant if the CI is entirely outside the configured SIGNIFICANT_IMPACT_THRESHOLD (or the deprecated UNCONFIDENCE_THRESHOLD).

For instance, for an execution time metric, this confidence interval indicates a significantly worse performance:

----------------------------------------|---------|---(---------^---------)---------->
                                       0%        1%  1.3%      2.2%      3.1%
                                                  |   |         |         |
       significant impact threshold --------------'   |         |         |
                      lower bound of CI --------------'         |         |
       sample mean (center of the CI) --------------------------'         |
                      upper bound of CI ----------------------------------'

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-29T09:44:38.081039Z	29b47bc	Draft marked ready
🔒 Security Review	✅ Completed 2026-09-29T09:45:11.591176Z	29b47bc	Draft marked ready

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[igoragoli]

Nits resolved! Thank you for the review @xlamorlette-datadog :)

[xlamorlette-datadog]

@igoragoli You forgot https://github.com/DataDog/dd-trace-cpp/pull/371/changes#r4155791504.

[xlamorlette-datadog]

@igoragoli I fixed it in an opened PR: 84fb898.