test: Add python backend tests for the new histogram metric

[yinggeh]

What does the PR do?

Tests histogram metric in custom_metrics.

Checklist

• [x] PR title reflects the change and is of format <commit_type>: <Title>
• [x] Changes are described in the pull request.
• [x] Related issues are referenced.
• [x] Populated github labels field
• [x] Added test plan and verified test passes.
• [x] Verified that the PR passes existing CI.
• [x] Verified copyright is correct on all changed files.
• [x] Added succinct git squash message before merging ref.
• [x] 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.

• [x] test

Related PRs:

https://github.com/triton-inference-server/vllm_backend/pull/56 https://github.com/triton-inference-server/python_backend/pull/374 https://github.com/triton-inference-server/core/pull/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

[rmccorm4]

Please trigger a pipeline encapsulating all the latest changes so can feel confident in the CI impact when looking at the cherry-picks

[yinggeh]

Merged https://github.com/triton-inference-server/server/pull/7525 to the wrong branch. Need re-approval to merge to main.