This CR adds the following raft-specific metrics:
	- cluster_size
	- is_leader
	- committed_block_number
	- snapshot_block_number
	- # leader_changes
	- # proposal_failures

Change-Id: Ie35f2984bc8eaecdd5826b1e1aa21e6c759fdd83
Signed-off-by: Adarsh Saraf <adarshsaraf123@gmail.com>