Add metrics for replication lag

[ikhoon]

We don't measure the time it takes for a command to be replicated to other replicas. If a revision is not found on a server, the replication lag could help diagnose the cause.

public final class ReplicationLog<T> {

    private final int replicaId;
    private final Command<T> command;
    private final T result;
    // Add a timestamp to compare with the local time of the replica server
    private final Instant createdAt;
    ...