feat: metrics

Monitor metrics:

- online clients
- message count
- subscribe count
- connect count

Look for existing metrics or telemetry code in the codebase, likely in a monitoring or stats module. The issue mentions tracking online clients, message count, subscribe count, and connect count. Start by finding where client connections and message handling are managed, then implement counters or gauges exposed via a metrics endpoint. Check if there's a preferred metrics library (like Prometheus) in use. Verify by running the broker and ensuring metrics are collected and accessible.

Written by the indexing model from the issue text.