feature: Design a Dedicated Metrics Module for Engine/Request Metrics

Describe the feature

Currently, metrics related to engine/request in the production-stack are collected within the router——a responsibility that should ideally belong to a dedicated observability layer, This architecture leads to:

1. Violation of separation of concerns: Router's core function is routing, not metrics management
2. Duplicate collection overhead: Both HPA and observability systems need to scrape metrics from router, creating unnecessary 2x load
3. Tight coupling: Metrics format changes require router modifications

Implementing centralized metrics storage would:

1. Enable single-point collection for all consumers (cluster obser #474, router scheduling, HPA #238)
2. Reduce router CPU/memory pressure by offloading metrics work
3. Standardize metrics access across subsystems

Why do you need this feature?

Design a dedicated metrics module/database (similar to etcd/redis) that would:

1. Collect and store all engine/request metrics centrally
2. Provide a unified interface for metrics querying
3. Support both real-time and historical metrics analysis

Additional context

Do you think a separate metrics module is necessary? I'm still learning about system work, so I'd be grateful for your corrections or suggestions!

No files, tests, or entry points are named. Start by locating the router's current engine/request metric collection and reading related issues #474 and #238; the issue does not define a concrete implementation or an unambiguous definition of done.