Add Consensus "follower lag" time metric

It would be useful to have metrics that measured the lag time between leader WAL writes and follower WAL writes. Imagine if a node on a cluster had a very slow disk or was extremely overloaded. That node may constantly be falling behind and/or remote bootstrapping. It would help to be able to monitor for nodes that were constantly very far behind the leader (high seconds or minutes) so that administrators could take a look at these slow machines and either remove them from the cluster or fix the underlying issues.