Health metric API to expose Redis connection pool metrics

[achimnol]

Let's add a Prometheus-compatible API endpoint to expose the health metrics of Redis connection pools used by each manager process.

• https://github.com/prometheus/docs/blob/main/content/docs/instrumenting/exposition_formats.md

The API handler itself should be very simple to implement. Since we have a multi-node multi-process architecture for Manager, we should use an external storage (Redis) to aggregate the metrics from different manager processes, and adopt a separate Redis connection mechanism like #2041 to avoid interference with the monitored connection pool.

The metric may be composed of:

• per-process connection pool information
  • per-pool metrics for different redis db by usage
  • e.g., the pool size, the occupancy
• redis-side metrics
  • e.g., number of total client connections

[achimnol]

👀 #2567 feat: Add manager stat API that compatible with Prometheus +221/-0
👀 #2566 feat: Add manager status report bgtask +48/-3
👀 #2565 feat: Introduce models.manager module that treats manager status +142/-0