chore(openai): add model tag for openai metrics for consistency with openai saas integration [backport 2.11]

[github-actions]

Backport 07ecb27 from #10282 to 2.11.

Metrics from the OpenAI SaaS integration (openai.api.usage...) have a model tag. We should also tag metrics from OpenAI APM Integration with model for convenience (for example if customers want to use both openai.api.usage... metrics and openai.ratelimit... metrics in the same dashboard).

Addresses #10185

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-10282-to-2.11
Commit report: 9503e3a
Test service: dd-trace-py

✅ 0 Failed, 795 Passed, 40209 Skipped, 15m 29.26s Total Time

[pr-commenter]

Benchmarks

Benchmark execution time: 2024-08-26 20:24:28

Comparing candidate commit 9503e3a in PR branch backport-10282-to-2.11 with baseline commit b95a737 in branch 2.11.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 214 metrics, 2 unstable metrics.

[github-actions]

CODEOWNERS have been resolved as:

releasenotes/notes/openai-model-tag-2482b3d5b2905db9.yaml               @DataDog/apm-python
ddtrace/llmobs/_integrations/openai.py                                  @DataDog/ml-observability
tests/contrib/openai/test_openai_v0.py                                  @DataDog/ml-observability
tests/contrib/openai/test_openai_v1.py                                  @DataDog/ml-observability