build(deps): update Bazel dependencies and pin Bazel 9.1.1

[dependabot]

Description

Updates the Bazel dependency group:

• abseil-cpp: 20260107.1 → 20260526.0
• rules_cc: 0.2.18 → 0.2.21

Pins Bazelisk to Bazel 9.1.1.

Motivation

Abseil 20260526.0 fails with Bazel 9 releases before 9.1.0. The pin keeps local and CI Bazelisk runs on a compatible release.

Additional Notes

Validated locally:

• bazelisk --bazelrc=.bazelrc.std build dd_trace_cpp
• bazelisk --bazelrc=.bazelrc.absl build dd_trace_cpp
• git diff --check

The benchmark report found no performance regressions.

Jira ticket: N/A

[datadog-datadog-prod-us1]

🎯 Code Coverage (details)
• Patch Coverage: 100.00%
• Overall Coverage: 91.75% (+0.00%)

_{This comment will be updated automatically if new data arrives.
🔗 Commit SHA: 1c7a5a5 | Docs | Datadog PR Page | Give us feedback!}

[pr-commenter]

Benchmarks

Benchmark execution time: 2026-07-30 19:03:54

Comparing candidate commit 1c7a5a5 in PR branch dependabot/bazel/bazel-packages-9e06492fea with baseline commit 45976fb 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]

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[chatgpt-codex-connector]

Avoid requiring an unpinned Bazel upgrade

In builds that run Bzlmod with Bazel <8.6.0 (or pre-9.1.0 on the 9.x line), Abseil 20260526.0 is a known-bad release because its upstream MODULE.bazel removed compatibility_level; Abseil's release notes say those Bazel versions fail and recommend upgrading Bazel. This repo does not pin Bazel, and the Bazel job in .github/workflows/dev.yml invokes bazelisk --bazelrc=... build dd_trace_cpp, so users or CI images that have not moved to Bazel 8.6+/9.1+ can hit module-resolution failures before compilation; either pin/require a compatible Bazel version with this bump or stay on the previous Abseil LTS.

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