Add monitoring capabilities

We should add monitoring capabilities of the UniFi Controller.

Several options:

• activate and expose the SNMP port by default
• use a Prometheus exporter (provide a docker-compose with both images, or a bundled all-in-one image for system which do not have compose, e.g. NAS boxes)
• use the WebAPI to gather the data and write that to a UDP protocol using a de-facto standard (e.g. InfluxDB line protocol, etc.)