Enhancement: Refresh devices

[PhilipMay]

The devices are only checked at startup. That means when one device is added or removed while the exporter is running - it will not detect this.

[ngosang]

They won't be detected. You have to restart the exporter. The most common use case is to shutdown the server to add the devices, so, the exporter is restarted too. If you have some use case with plug & play and you need to refresh the devices, you could open a PR but it will be accepted only if the "refresh devices" could be disabled with an environment variable.