alerting: server should report statistics to Prometheus

Implement the ability in Stork server to export statistics to Prometheus. For the time being I think the following stats should be made available:

1. number of authorized machines
2. number of unauthorized machines
3. number of unreachable machines

The third statistic should report 0 under normal circumstances, so not exactly exciting. The idea is that an admin could set up an alarm for non-zero value reported. For the first two, a careful admin may set up an alert if there's a new machine in the system, e.g. when someone tries to set up a fake machine or when the system is scaled up by adding new VM with Kea.

In the future we may also report the number of error events or something similar. The general idea here is to offload the alterting to prometheus. We were supposed to provide some alterting mechanisms, but instead of implementing them ourselves in Stork, it's much more robust to use existing mechanisms from Prometheus.