fix(llmobs): recreate eval metric writer on fork [backport 2.13]

[github-actions]

Backport 5dbd7ef from #10710 to 2.13.

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

[datadog-dd-trace-py-rkomorn]

Datadog Report

Branch report: backport-10710-to-2.13
Commit report: a6a86ee
Test service: dd-trace-py

✅ 0 Failed, 955 Passed, 0 Skipped, 14m 30.1s Total Time

[pr-commenter]

Benchmarks

Benchmark execution time: 2024-09-23 20:54:55

Comparing candidate commit a6a86ee in PR branch backport-10710-to-2.13 with baseline commit efb3c0e in branch 2.13.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 353 metrics, 47 unstable metrics.