Add Prometheus support v2

[SchumacherFM]

This is a refactored version of #195 by @kereis

• Updates to the status screen of the webinterface to add model and SN
• Remove too many prometheus measurements like: monitoring websocket connections, modbus connections but keeps of course all modbus measured values
• some other minor refactorings

This PR runs at the moment at my home with SolarEdge SE10K, SE17K and the Meter.

IMHO #195 can be closed.

Waiting for your review(s).