[Store] Add OpLog writer runtime metrics

[Icedcoco]

Description

Expose a single writer runtime snapshot through HA metrics and admin outputs, covering admission, queues, retries, durability, errors, stuck ranges, and terminal reason.

Module

• mooncake-store

Type of change

• Feature

Testing

• pre-commit run --files on all changed files
• git diff --check
• Full CMake build blocked because extern/pybind11 is absent in this checkout.

Checklist

• Focused diff
• Added unit coverage for admission snapshot

AI assistance

AI-assisted implementation; human submitter must review every changed line.