feat(traces): [SVLS-8734] respect Datadog-Client-Computed-Stats header

[lym953]

Background

traces/stats that go through Lambda extension can generate stats in three ways:

1. tracer generates stats and sends them to extension at /v0.6/stats. Only works if the env var DD_TRACE_STATS_COMPUTATION_ENABLED is true.
2. extension generates stats from traces. It's off by default, and works only if env var DD_COMPUTE_TRACE_STATS_ON_EXTENSION is true.
3. Datadog backend generates stats from traces. Works only if the trace tag _dd.compute_stats is 1.

If a trace has the header Datadog-Client-Computed-Stats with a truthy value, then that means 1 is on, so we need to turn off 2 and 3 to avoid duplicate counts, i.e.

1. make extension not generate stats
2. set trace tag _dd.compute_stats to 0

Summary

• When the Datadog-Client-Computed-Stats header is truthy, which means the tracer has already computed stats, the extension skips its own stats generation (turns off option 2)
• Also sets _dd.compute_stats to 0 in trace payload tags so the backend skips stats computation too (turns off option 3)
• Made COMPUTE_STATS_KEY pub(crate) so it can be referenced from trace_processor

Test plan

Automated tests

Passed the new unit tests

Manual tests

Steps

1. Deploy a Lambda function that uses github.com/DataDog/dd-trace-go/v2 v2.6.0
2. Set env var:
3. DD_TRACE_STATS_COMPUTATION_ENABLED: true, which turns on 1
4. DD_COMPUTE_TRACE_STATS_ON_EXTENSION: true, which tries to turn on 2

Result

Before: 2 hits (duplicate), though with different resource_name tag

After: TODO

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Updates the Lambda extension trace pipeline to respect the Datadog-Client-Computed-Stats header so stats are not double-computed (by the extension and/or backend) when the tracer already computed them.

Changes:

• When client_computed_stats is true, forces _dd.compute_stats to "0" in trace payload tags so backend stats computation is disabled.
• Skips extension-side stats generation when client_computed_stats is true, even if DD_COMPUTE_TRACE_STATS_ON_EXTENSION is enabled.
• Makes COMPUTE_STATS_KEY pub(crate) and adds unit tests covering the new behavior.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
bottlecap/src/traces/trace_processor.rs	Overrides _dd.compute_stats when the tracer computed stats; skips extension stats generation; adds tests.
bottlecap/src/tags/lambda/tags.rs	Exposes COMPUTE_STATS_KEY within the crate for reuse.

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[Copilot]

This test’s doc comment says it verifies send_processed_traces skips stats generation when client_computed_stats is true, but the config in this test has compute_trace_stats_on_extension: false, so stats generation is skipped regardless of the header. Consider removing the send_processed_traces portion here (the following test already covers the header-based skip), or adjust the setup so this test actually exercises the new condition.