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

Files changed

• include/datadog/environment.h — Added _DD_ROOT_CPP_SESSION_ID to DD_LIST_ENVIRONMENT_VARIABLES registry; added environment::set() declaration
• src/datadog/environment.cpp — Added environment::set() implementation (platform-aware, no-overwrite)
• include/datadog/tracer_signature.h — Added root_session_id member to TracerSignature
• src/datadog/tracer.cpp — Computes root session ID via environment::lookup; registers it via environment::set for exec propagation
• src/datadog/telemetry/telemetry_impl.cpp — Adds session headers to app_started() and send_payload()
• supported-configurations.json — Added _DD_ROOT_CPP_SESSION_ID entry
• Tests — Session header validation: root process (no DD-Root-Session-ID), child process (DD-Root-Session-ID present), heartbeat includes headers

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

Test plan

• All existing assertions pass (107 test cases)
• New test: root process sends DD-Session-ID only
• New test: child process sends both DD-Session-ID and DD-Root-Session-ID
• New test: heartbeat includes session headers

🤖 Generated with Claude Code

[pr-commenter]

Benchmarks

Benchmark execution time: 2026-03-24 19:26:09

Comparing candidate commit a3b7684 in PR branch ayan.khan/stable-session-id-headers with baseline commit 14280af 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 [+3.152ms; +3.361ms] or [+4.160%; +4.435%]