perf: vLLM metrics optimization

[yinggeh]

What does the PR do?

Optimize vLLM by reporting metrics in a separate thread.

Checklist

• PR title reflects the change and is of format <commit_type>: <Title>
• Changes are described in the pull request.
• Related issues are referenced.
• Populated github labels field
• Added test plan and verified test passes.
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Commit Type:

Check the conventional commit type
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• perf

Related PRs:

triton-inference-server/vllm_backend#66

Where should the reviewer start?

n/a

Test plan:

L0_backend_vllm/metrics_test

• CI Pipeline ID:
  18506139

Caveats:

n/a

Background

n/a

Related Issues: (use one of the action keywords Closes / Fixes / Resolves / Relates to)

n/a