feat: add raft member lag

[liebman]

Adding metrics for how far behind the leader each raft member is

# HELP consul_raft_lag How far behind the leader a node is.
# TYPE consul_raft_lag gauge
consul_raft_lag{member="consul1",member_id="xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxx0"} 1
consul_raft_lag{member="consul2",member_id="xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxx1"} 0
consul_raft_lag{member="consul3",member_id="xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxx2"} 1
consul_raft_lag{member="consul4",member_id="xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxx3"} 2
consul_raft_lag{member="consul5",member_id="xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxx4"} 0