fix(llmobs): propagate distributed headers via signal dispatching, not config [forwardport 3.x]

[erikayasuda]

Forwardporting #12089

This PR makes a change to our shared distributed tracing header injection method to dispatch signals/events instead of relying on the global config settings, which is only modifiable via env vars. This fixes distributed tracing for users that might rely solely on the LLMObs.enable() setup config.

Programmatic LLMObs.enable()/disable() calls do not set the global config._llmobs_enabled boolean setting, which is only controlled by the DD_LLMOBS_ENABLED env var. This was problematic for users that relied on manual LLMObs.enable() setup (i.e. no env vars) because our distributed tracing injection code only checks the global config to inject llmobs parent IDs into request headers. If users manually enabled LLMObs without any env vars, then this would not be reflected in the global config value and thus LLMObs parent IDs would never be injected into the request headers.

We can't check directly if LLMObs is enabled in the http injection module because:

This would require us to import significant product-specific LLMObs-code into the shared http injector helper module which would impact non-LLMObs users' app performance
Circular imports in LLMObs which imports http injector logic to use in its own helpers
Instead of doing our check based on the global config._llmobs_enabled setting, we now send a tracing event to our shared product listeners, and register a corresponding LLMObs._inject_llmobs_context() hook to be called for all inject() calls if LLMObs is enabled (we check the LLMObs instance, not the global config setting value).

One risk and why I don't like changing global config settings is because this then implies that it is no longer global or tied to an env var (I want to push for env var configuration where possible over manual overriding/enabling). If a global enabled config can be toggled indiscriminately then this could open a can of worms for enabling/disabling logic in our LLMObs service, which isn't really designed to be toggled on/off multiple times in the app's lifespan. However if some users cannot rely on env vars, then I don't see any other solution that does not couple tracer internal code with LLMObs code which is a no-option. (UPDATE: we avoided this issue by using signal dispatching)

Checklist

• PR author has checked that all the criteria below are met
• The PR description includes an overview of the change
• The PR description articulates the motivation for the change
• The change includes tests OR the PR description describes a testing strategy
• The PR description notes risks associated with the change, if any
• Newly-added code is easy to change
• The change follows the library release note guidelines
• The change includes or references documentation updates if necessary
• Backport labels are set (if applicable)

Reviewer Checklist

• Reviewer has checked that all the criteria below are met
• Title is accurate
• All changes are related to the pull request's stated goal
• Avoids breaking API changes
• Testing strategy adequately addresses listed risks
• Newly-added code is easy to change
• Release note makes sense to a user of the library
• If necessary, author has acknowledged and discussed the performance implications of this PR as reported in the benchmarks PR comment
• Backport labels are set in a manner that is consistent with the release branch maintenance policy

[datadog-dd-trace-py-rkomorn]

Datadog Report

Branch report: backport-12089-to-3.x
Commit report: 423e0e1
Test service: dd-trace-py

✅ 0 Failed, 130 Passed, 1378 Skipped, 5m 26.28s Total duration (35m 9.35s time saved)

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-01-30 11:48:39

Comparing candidate commit 423e0e1 in PR branch backport-12089-to-3.x with baseline commit 09517b8 in branch 3.x-staging.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 394 metrics, 2 unstable metrics.

[github-actions]

CODEOWNERS have been resolved as:

releasenotes/notes/fix-llmobs-enable-updates-config-45379a7a30e2e0e3.yaml  @DataDog/apm-python
ddtrace/llmobs/_llmobs.py                                               @DataDog/ml-observability
ddtrace/propagation/http.py                                             @DataDog/apm-sdk-api-python
tests/llmobs/test_llmobs_service.py                                     @DataDog/ml-observability
tests/llmobs/test_propagation.py                                        @DataDog/ml-observability
tests/tracer/test_propagation.py                                        @DataDog/apm-sdk-api-python