Initialize Prometheus metrics on node start where appropriate and review
the usage of the following metrics:
```
anna@kiwi:~/Documents/GitProjects/nspcc-dev/neo-go$ find | grep prometheus.go
./pkg/network/prometheus.go
./pkg/core/stateroot/prometheus.go
./pkg/core/prometheus.go
./pkg/services/rpcsrv/prometheus.go
./pkg/services/metrics/prometheus.go
```
Close#2970.
Signed-off-by: Anna Shaleva <shaleva.ann@nspcc.ru>
97b93c6833