fix(traces): keep backend as safety net for trace stats

[duncanista]

Summary

Fixes trace stats "disappearing" after upgrading to v95.

PR #1118 (55659d9) introduced logic to respect the Datadog-Client-Computed-Stats header from tracers. When a tracer sends this header as truthy, the extension would:

1. Skip extension-side stats generation (correct)
2. Set _dd.compute_stats: 0 on the trace payload, telling the backend to also skip stats (problematic)

This created a scenario where all three stats sources could be disabled simultaneously:

DD_COMPUTE_TRACE_STATS_ON_EXTENSION	Datadog-Client-Computed-Stats	_dd.compute_stats	Ext generates?	Result
false	true	"0"	No	No stats from anyone if tracer stats don't reach backend

If the tracer claims it computed stats but those stats don't actually reach the backend (tracer bug, misconfiguration, or the header is sent without stats being computed), stats vanish entirely. In v94, the backend always acted as a safety net (_dd.compute_stats: 1).

Fix

_dd.compute_stats is now determined solely by compute_trace_stats_on_extension, ignoring client_computed_stats:

DD_COMPUTE_TRACE_STATS_ON_EXTENSION	Datadog-Client-Computed-Stats	_dd.compute_stats	Ext generates?
false	false	"1"	No
false	true	"1" (was "0")	No
true	false	"0"	Yes
true	true	"0"	No (dedup preserved)

The client_computed_stats header still correctly suppresses extension-side stats generation (the && !client_computed_stats guard on stats_generator.send()), preserving the dedup behavior from #1118 when DD_COMPUTE_TRACE_STATS_ON_EXTENSION=true.

Worst case with this fix is double-counted stats (tracer + backend) — far better than zero stats.

Test plan

• Unit tests updated and passing (check_compute_stats_behavior covers all 4 combinations)
• Build a layer from this branch and deploy to affected customer's Lambda
• Verify trace stats reappear in Datadog UI

[Copilot]

Pull request overview

This PR fixes a critical bug in trace stats generation where stats could disappear after upgrading to v95. The issue was introduced in PR #1118, where the extension would set _dd.compute_stats: "0" when a tracer claimed to have computed stats via the Datadog-Client-Computed-Stats header. If those tracer stats didn't reach the backend (due to tracer bugs or misconfiguration), stats would vanish entirely with no fallback.

The fix changes _dd.compute_stats to be determined solely by compute_trace_stats_on_extension, ignoring client_computed_stats. This keeps the backend as a safety net that can always compute stats if needed, while client_computed_stats continues to prevent duplicate stats from the extension (when compute_trace_stats_on_extension=true).

Changes:

• Updated _dd.compute_stats determination logic in trace_processor.rs to depend only on compute_trace_stats_on_extension
• Updated all four test cases and their expectations to match the new behavior
• Bumped extension version from "94-next" to "95-next"

Reviewed changes

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

File	Description
bottlecap/src/traces/trace_processor.rs	Updated compute_stats logic and all test expectations
bottlecap/src/tags/lambda/tags.rs	Bumped extension version to 95-next

[lym953]

LGTM as a remediation of missing trace stats issue. But not sure if it will cause double counting, which was what #1118 was trying to fix. Leaving to APM Serverless team to evaluate the possibility of double counting.

[duncanista]

Drafting due to #1172

[duncanista]

Also failing internal testing.