fix(llmobs): set session ID on span if annotated as a tag

[Yun-Kim]

This PR makes a fix to how session ID is set on a LLM Obs span.

Session ID (for session tracking) can be explicitly set when starting a span via LLMObs._start_span(session_id=...). We then set this both as the span's session_id field and as a tag on the span, "session_id:...".

However, currently session ID can also be set by using LLMObs.annotate/annotation_context() as a tag. This is meant for use cases where the session ID may not be immediately available. The issue here is that when setting as a tag, we don't actually set that value on the span's top level session_id field and only set it as a tag. This means that our backend and other event queries made by other products (like RUM/APM) can't 100% rely on that top level event field even if a user did indeed set it as a tag. (queries based on tags are also slower/more expensive than top-level queries)

This fixes the issue by adding a step when setting tags via LLMObs.annotate/annotation_context() that we also add a session_id tag as the span's top-level session_id field.

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-session-id-via-tags-6f6b0f7fca47e516.yaml  @DataDog/apm-python
ddtrace/llmobs/_llmobs.py                                               @DataDog/ml-observability
tests/llmobs/test_llmobs_service.py                                     @DataDog/ml-observability

[github-actions]

Bootstrap import analysis

Comparison of import times between this PR and base.

Summary

The average import time from this PR is: 231 ± 4 ms.

The average import time from base is: 234 ± 3 ms.

The import time difference between this PR and base is: -2.9 ± 0.2 ms.

Import time breakdown

The following import paths have shrunk:

ddtrace.auto 2.012 ms (0.87%)

ddtrace.bootstrap.sitecustomize 1.334 ms (0.58%)

ddtrace.bootstrap.preload 1.334 ms (0.58%)

ddtrace.internal.products 1.334 ms (0.58%)

ddtrace.internal.remoteconfig.client 0.616 ms (0.27%)

ddtrace 0.678 ms (0.29%)

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-04-16 21:13:49

Comparing candidate commit 383a246 in PR branch yunkim/llmobs-fix-session-id with baseline commit dc4cd33 in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 506 metrics, 2 unstable metrics.

[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-13220-to-2.21
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 bdca42d6005262a2bf803f8e928f8ae5f6daec38
# Push it to GitHub
git push --set-upstream origin backport-13220-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-13220-to-2.21.