Ops: Implement consensus check

[msbrogli]

Design: #98

This task also includes adding a metric to be exported by Prometheus.

[jansegre]

I think Luis has implemented something like this already. Can you confirm? @luislhl

[jansegre]

According to Luis this isn't exactly implemented, although monitoring has improved significantly. There's still some improvement that can be made when checking if the consensus from different nodes has drifted. But it's probably purely on Ops side now, since there's enough APIs to do this.