MQTT Performance Metrics Management

[OliverHennhoefer]

Ephemeral vs Persistent Metrics

Current State: Session-only metrics that reset on service restart
Question: Should these be persistent for historical analysis?

In backend/src/off_key/services/mqtt/charger_discovery.py

[seffs]

@OliverHennhoefer Is this still relevant?

[OliverHennhoefer]

@seffs I think we can skip this for now.