fix(llmobs): toggle config when enabled/disabled programmatically

[Yun-Kim]

This PR makes a change to programmatic LLMObs.enable()/disable() calls to explicitly change the value of the global config._llmobs_enabled boolean setting, which was previously 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:

1. 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
2. Circular imports in LLMObs which imports http injector logic to use in its own helpers

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.

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

[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
tests/llmobs/test_llmobs_service.py                                     @DataDog/ml-observability

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-01-25 00:22:44

Comparing candidate commit 0d35665 in PR branch yunkim/llmobs-fix-enable with baseline commit 1811db1 in branch main.

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