feat: Add vLLM metrics (counter and gauge) access through Triton

[yinggeh]

What does the PR do?

Add vLLM metrics (counter and gauge) access through python_backend custom metrics.

Checklist

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Commit Type:

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• feat

Related PRs:

triton-inference-server/vllm_backend#53
triton-inference-server/server#7493

Where should the reviewer start?

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Test plan:

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• CI Pipeline ID:
  17273623

Caveats:

Background

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