test: Test histogram metric

[yinggeh]

What does the PR do?

Tests histogram metric in custom_metrics.

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.
• Verified that the PR passes existing CI.
• Verified copyright is correct on all changed files.
• Added succinct git squash message before merging ref.
• All template sections are filled out.
• Optional: Additional screenshots for behavior/output changes with before/after.

Commit Type:

Check the conventional commit type
box here and add the label to the github PR.

• test

Related PRs:

triton-inference-server/vllm_backend#56
triton-inference-server/python_backend#374
triton-inference-server/core#386

Where should the reviewer start?

n/a

Test plan:

n/a

• CI Pipeline ID:
  17487728

Caveats:

n/a

Background

Customer requested histogram metrics from vLLM.

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

n/a