fix(llmobs): avoid missing ml app error if llmobs is disabled

[Yun-Kim]

Our _start_span() method does a bunch of llmobs-instrumentation/modifications under the hood, even if LLMObs is disabled. Specifically our llm/task/tool/agent/retrieval/workflow/embeddings() methods only log a warning if ML_APP is missing, but still route to our _start_span() method, which then continues on to do a ML_APP check (and raises if this is missing), even if LLMObs is disabled. This means if ML_APP is missing, any manual instrumentation will break customer apps if they disable LLMObs.

We need to make _start_span() a no-op wrapper around tracer.trace(...). This fix ensures that if llmobs is disabled, we return the span immediately before we start doing llmobs-specific logic.

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-ml-app-disabled-7879e1fcb2d1e0da.yaml     @DataDog/apm-python
ddtrace/llmobs/_llmobs.py                                               @DataDog/ml-observability
tests/llmobs/test_llmobs_service.py                                     @DataDog/ml-observability

[sabrenner]

lgtm just a release note suggestion

[github-actions]

Bootstrap import analysis

Comparison of import times between this PR and base.

Summary

The average import time from this PR is: 280 ± 3 ms.

The average import time from base is: 284 ± 5 ms.

The import time difference between this PR and base is: -3.9 ± 0.2 ms.

Import time breakdown

The following import paths have shrunk:

ddtrace.auto 2.133 ms (0.76%)

ddtrace.bootstrap.sitecustomize 1.449 ms (0.52%)

ddtrace.bootstrap.preload 1.449 ms (0.52%)

ddtrace.internal.remoteconfig.client 0.677 ms (0.24%)

ddtrace 0.685 ms (0.24%)

ddtrace.internal._unpatched 0.034 ms (0.01%)

json 0.034 ms (0.01%)

json.decoder 0.034 ms (0.01%)

re 0.034 ms (0.01%)

enum 0.034 ms (0.01%)

types 0.034 ms (0.01%)

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-06-18 21:49:25

Comparing candidate commit 80fb0cd in PR branch yunkim/llmobs-fix-start-span-disabled with baseline commit 55dd9fb in branch main.

Found 0 performance improvements and 3 performance regressions! Performance is the same for 558 metrics, 3 unstable metrics.

scenario:iastaspectsospath-ospathbasename_aspect

• 🟥 execution_time [+374.151ns; +604.242ns] or [+8.807%; +14.224%]

scenario:iastaspectsospath-ospathsplitext_aspect

• 🟥 execution_time [+521.396ns; +641.090ns] or [+11.532%; +14.179%]

scenario:iastaspectssplit-splitlines_aspect

• 🟥 execution_time [+102.951ns; +136.888ns] or [+7.007%; +9.316%]

[github-actions]

The backport to 2.21 failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-2.21 2.21
# Navigate to the new working tree
cd .worktrees/backport-2.21
# Create a new branch
git switch --create backport-13717-to-2.21
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 17ce936ee286f1b66116843126854a52504b235f
# Push it to GitHub
git push --set-upstream origin backport-13717-to-2.21
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-2.21

Then, create a pull request where the base branch is 2.21 and the compare/head branch is backport-13717-to-2.21.