Implement metrics - rate_replication, bandwidth_usage

Change-Id: Ic57d649ba9a46ccf5b464c39c6f153215d215f0c
Reviewed-on: http://review.couchbase.org/44252
Reviewed-by: Yu Sui <ysui68@gmail.com>
Tested-by: Xiaomei Zhang <xiaomeizhangatwork@gmail.com>