Add Prometheus node services metrics

[AnnaShaleva]

Ref. #3229. We need to add common Prometheus metrics to our network server (we already have a separate Service interface with Name() included), and the metric itself should denote whether specified service is enabled or not.

After that we need to include custom service-specific Prometheus metric where every service can decide what to include into this metric. One of the things that must be included into wallet-required services is whether service is authorized to sign anything at the current moment, i.e. whether it has a designated account unlocked (this metric is relevant for Notary service, Oracle service and etc.).