feat(tracing): add OpenTelemetry tracestate sampling

[MilanGarnier]

Summary

Adds W3C tracestate support for OpenTelemetry ot sampling state.

• Parses ot separately from Datadog and other vendor state.
• Retains valid inherited state, drops malformed rv and th values, and writes sampling values for eligible local probability decisions.
• Carries W3C state only when it belongs to the selected matching trace.

Testing

• cmake --build .build -j
• ./.build/test/tests

[pr-commenter]

Benchmarks

Benchmark execution time: 2026-08-20 12:29:05

Comparing candidate commit 6eb3ba3 in PR branch milan.garnier/otel-tracestate-ot-th with baseline commit 765983d in branch main.

Found 3 performance improvements and 2 performance regressions! Performance is the same for 3 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 ----------------------------------'

scenario:BM_HexPadded_uint64/NoPadding

• 🟥 execution_time [+1.272ns; +1.332ns] or [+2.427%; +2.543%]

scenario:BM_TraceID_HexPadded/NoPadding

• 🟥 execution_time [+4.020ns; +4.233ns] or [+2.029%; +2.137%]

scenario:BM_TraceID_ParseHex/128bit

• 🟩 execution_time [-37.427ns; -37.342ns] or [-21.666%; -21.617%]

scenario:BM_TraceID_ParseHex/64bit

• 🟩 execution_time [-17.590ns; -17.553ns] or [-19.169%; -19.128%]

scenario:BM_TraceTinyCCSource

• 🟩 execution_time [-2.674ms; -2.396ms] or [-3.456%; -3.096%]

[xlamorlette-datadog]

Mostly nit and opt comments.

[xlamorlette-datadog]

nit: extract this last condition (*decision.probability_sampled && decision.priority <= 0) into a named variable