Feature description

A new metric that exposes the connection state of each device as a gauge value (e.g., 0 = disconnected; 1 = connected, ...).

Problem or use case

I have an android device where the operating system frequently kills the syncthing app.
I want to monitor, using prometheus, whether the device is connected with my central syncthing node and get alerted if it is disconnected for an extended period of time.

Alternatives or workarounds

Create a custom exporter that uses the syncthing API.

If this is something you would be interested into having, I can try to open a PR for it.