feat(config): add DD_SERVERLESS_APM_ONLY traces-only mode

[zarirhamza]

Summary

Adds a single master switch — DD_SERVERLESS_APM_ONLY — that puts the extension in APM-only ("traces only") mode. When enabled, every billable metrics and logs egress path is suppressed so the customer incurs no infrastructure-monitoring or log-ingestion charges, while traces and APM trace stats are unaffected.

This closes the gap that made a true "traces only" configuration impossible before: there was no toggle to disable custom DogStatsD metrics, so even with logs and enhanced metrics turned off, custom metrics could still egress. A single, defensive flag now guarantees the behavior.

• Default is false — existing behavior is completely preserved.
• Readable from both environment (DD_SERVERLESS_APM_ONLY) and YAML.

Behavior

Signal	APM-only mode
Traces / APM trace stats	sent (unchanged)
Enhanced metrics	suppressed
Process (proc) enhanced metrics	suppressed
Custom DogStatsD metrics	suppressed
OTLP metrics	suppressed
Logs (incl. OTLP logs)	suppressed

Enforcement (defense in depth)

The flag is enforced at multiple layers so the guarantee holds regardless of other configuration (e.g. even if a user also sets DD_ENHANCED_METRICS=true):

1. Config (config/mod.rs) — forces serverless_logs_enabled, enhanced_metrics, lambda_proc_enhanced_metrics, otlp_config_metrics_enabled, and otlp_config_logs_enabled to false, overriding any individually set toggles.
2. Metrics wiring (main.rs::start_metrics_flushers) — returns no flushers, so anything drained from the DogStatsD aggregator (custom / enhanced / process) is discarded rather than sent.
3. Logs flusher (logs/flusher.rs::flush) — short-circuits to an empty request set, guaranteeing no log egress even if something is queued or redriven.

Testing

• cargo fmt --all -- --check — passes.
• cargo clippy --workspace --all-targets --features default — passes (no new warnings).
• cargo test (config + logs modules) — 55 passed, including two new tests:
  • test_serverless_apm_only_defaults_off — flag defaults off; other toggles unchanged.
  • test_serverless_apm_only_forces_metrics_and_logs_off — flag overrides explicitly-enabled metrics/logs toggles.

Live validation on AWS Lambda

Built the layer (arm64) and deployed two otherwise-identical Node.js 22 functions (Datadog Node layer + this extension), one with the flag off and one on, then invoked each and queried the Datadog API:

Signal	flag OFF	flag ON (APM-only)
Spans / traces	present	present
Logs	present	0
Custom metric	present	absent (never ingested)

The extension's Lambda Telemetry API subscription also drops from [Platform, Extension, Function] to [Platform] when the flag is on, confirming logs are cut off at the source.

Risk / rollback

Low. Behavior is gated entirely behind a new flag that defaults to false; when unset the code paths are unchanged. Rollback is setting/leaving DD_SERVERLESS_APM_ONLY unset.

Notes for reviewers

• The FIPS-feature clippy variant could not be run locally (macOS is missing the aws-lc FIPS crypto dylib); it exercises the same feature-agnostic config/logs code and runs on the Ubuntu CI runners.

[datadog-prod-us1-3]

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⚠️ Warnings

🚦 3 Pipeline jobs failed

DataDog/datadog-lambda-extension | integration-suite: [lmi]     

DataDog/datadog-lambda-extension | integration-suite: [on-demand]     

DataDog/datadog-lambda-extension | integration-suite: [snapstart]     

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