Implement stable session identifier headers for telemetry

[khanayan123]

Summary

Implements the Stable Service Instance Identifier RFC for the C++ SDK.

• Adds DD-Session-ID (= runtime_id) and conditional DD-Root-Session-ID headers to all telemetry requests (app-started, heartbeats, app-closing, etc.)
• DD-Root-Session-ID is only sent when it differs from DD-Session-ID (i.e., in child processes)
• Root session ID is read from _DD_ROOT_CPP_SESSION_ID via the config registry (environment::lookup) or defaults to the current runtime_id
• environment::set(_DD_ROOT_CPP_SESSION_ID, ...) is called at tracer init so exec'd children inherit it automatically (no-op if already set)
• Fork propagation is automatic — memory survives fork, so the root session ID persists in child processes

Integration testing note

System-tests for cpp_nginx and cpp_httpd cannot validate session headers until the downstream modules (nginx-datadog, httpd-datadog) release a version that includes these dd-trace-cpp changes. The system-tests manifest entries remain missing_feature until then. Unit tests in this PR should validate the implementation directly.

Companion system-tests PR: DataDog/system-tests#6510

[pr-commenter]

Benchmarks

Benchmark execution time: 2026-03-26 19:14:04

Comparing candidate commit f124ca3 in PR branch ayan.khan/stable-session-id-headers with baseline commit 910e3d5 in branch main.

Found 0 performance improvements and 1 performance regressions! Performance is the same for 0 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_TraceTinyCCSource

• 🟥 execution_time [+3.745ms; +3.941ms] or [+4.997%; +5.259%]

[datadog-prod-us1-6]

🎯 Code Coverage (details)
• Patch Coverage: 83.87%
• Overall Coverage: 90.83% (-0.00%)

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