fix(llmobs): recreate eval metric writer on fork

[lievan]

This PR ensures that the LLMObsEvalMetricWriter is correctly recreated and restarted on a forked process.

Previously, on a process fork we were not recreating/restarting the eval metric writer worker.

Mirrors #10249 but for eval metric writer

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-forked-eval-metric-writer-55d50d1ed8fc10ea.yaml  @DataDog/apm-python
ddtrace/llmobs/_llmobs.py                                               @DataDog/ml-observability
ddtrace/llmobs/_writer.py                                               @DataDog/ml-observability
tests/llmobs/test_llmobs_service.py                                     @DataDog/ml-observability

[pr-commenter]

Benchmarks

Benchmark execution time: 2024-09-20 15:48:46

Comparing candidate commit a45a282 in PR branch evan.li/eval-metric-fork with baseline commit baac738 in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 356 metrics, 48 unstable metrics.

[emmettbutler]

I'll wait to review until existing comments are addressed and tests are passing. Please re-request a review from me at that time. Thanks!

[Yun-Kim]

Minor nits, but LGTM

[github-actions]

The backport to 2.14 failed:

The process '/usr/bin/git' failed with exit code 128

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-2.14 2.14
# Navigate to the new working tree
cd .worktrees/backport-2.14
# Create a new branch
git switch --create backport-10710-to-2.14
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 5dbd7ef543031f50dd6600ea0af8b28773d56fc4
# Push it to GitHub
git push --set-upstream origin backport-10710-to-2.14
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-2.14

Then, create a pull request where the base branch is 2.14 and the compare/head branch is backport-10710-to-2.14.