[RFC] General extension mechanism for system metrics

## Issue Description

Type: *feature request*

### Describe what happened (or what feature you want)

Introduce a general extension mechanism for system metrics, so that all modules could observe the events when system metrics have changed, then leverage the metrics to perform strategy optimization (e.g. training for adaptive strategy).

The issue names no files, tests, or entry points. Start by locating the existing system-metrics collection and notification paths, then determine how modules currently consume metric changes. Done would require an agreed extension design and implementation scope, which the issue does not specify.

Written by the indexing model from the issue text.