feat(telemetry): implement app-extended-heartbeat event

[khanayan123]

Summary

Implement the `app-extended-heartbeat` telemetry event for the C++ tracer, scoped to the `configuration` payload only.

Motivation

Long-running services (24h+) currently only report their configuration state via the initial `app-started` event. If the backend misses or loses that event, there's no way to recover visibility into the SDK's configuration. The `app-extended-heartbeat` event solves this by re-sending the full configuration payload every 24h, ensuring reliable state reporting for long-running instances.

Per the spec, `integrations` and `dependencies` are optional — this PR scopes to `configuration` only, which is the focus of Config Visibility reliability.

Implementation

• The event fires periodically (default 24h) and includes the full `configuration` payload
• Configuration is tracked via `all_configurations_` — a map updated whenever `generate_configuration_field` is called — so the payload reflects runtime changes (e.g., remote config) not just static startup state
• Seq-ids are read from the existing `config_seq_ids_` map without incrementing, so extended heartbeat does not corrupt sequence numbering
• The interval is configurable via `DD_TELEMETRY_EXTENDED_HEARTBEAT_INTERVAL` (decimal seconds, default 86400) for system test parity validation

Changes

• `include/datadog/environment.h`: Declare `DD_TELEMETRY_EXTENDED_HEARTBEAT_INTERVAL` env var (DECIMAL, default 86400.0)
• `include/datadog/telemetry/configuration.h`: Add `extended_heartbeat_interval_seconds` (`Optional`) to `Configuration` and `extended_heartbeat_interval` to `FinalizedConfiguration`
• `src/datadog/telemetry/configuration.cpp`: Parse env var and finalize interval with validation
• `src/datadog/telemetry/telemetry_impl.h`: Add `all_configurations_` member; declare `extended_heartbeat_payload()`
• `src/datadog/telemetry/telemetry_impl.cpp`: Populate `all_configurations_` in `generate_configuration_field`; schedule recurring extended heartbeat task; build payload from `all_configurations_` with current seq-ids (no increment)
• `supported-configurations.json`: Updated supported configurations manifest
• `test/telemetry/test_configuration.cpp`: Test for env var parsing and default value
• `test/telemetry/test_telemetry.cpp`: Test that extended heartbeat includes configuration payload

Related

• System test PR: feat(telemetry): add app-extended-heartbeat parametric tests system-tests#6338

[pr-commenter]

Benchmarks

Benchmark execution time: 2026-03-31 05:04:20

Comparing candidate commit 6766649 in PR branch ayan.khan/add-extended-heartbeat 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 [+3.079ms; +3.510ms] or [+4.050%; +4.618%]

[datadog-prod-us1-6]

🎯 Code Coverage (details)
• Patch Coverage: 91.89%
• Overall Coverage: 90.89% (+0.06%)

_{This comment will be updated automatically if new data arrives.
🔗 Commit SHA: c888b84 | Docs | Datadog PR Page | Was this helpful? React with 👍/👎 or give us feedback!}

[anna-git]

thanks for adressing comments, nice work!