test: Test model_metrics config and document histogram buckets override

[yinggeh]

What does the PR do?

Tests new model_metrics message in pbtxt. Add example to override histogram buckets per-family.

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.
• [ ] Verified that the PR passes existing CI.
• [x] 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.

• [x] docs
• [x] test

Related PRs:

https://github.com/triton-inference-server/core/pull/405 https://github.com/triton-inference-server/common/pull/126

Where should the reviewer start?

Test plan:

L0_metrics

• CI Pipeline ID: 20106057

Caveats:

Background

Default histogram buckets does not satisfy all use cases.

[yinggeh]

Verified CI has completed without introducing new failures. @rmccorm4 @GuanLuo