Documentation: explain lease expiration metrics

Signed-off-by: Gyu-Ho Lee <gyuhox@gmail.com>

Documentation/metrics.md

@@ -18,7 +18,7 @@ Metrics that are etcd2 related are documented [v2 metrics guide][v2-http-metrics
 
 These metrics describe the status of the etcd server. In order to detect outages or problems for troubleshooting, the server metrics of every production etcd cluster should be closely monitored.
 
-All these metrics are prefixed with `etcd_server_`
+All these metrics are prefixed with `etcd_server_`, except `lease_expired_total` with `etcd_debugging_server_`
 
 | Name                      | Description                                              | Type    |
 |---------------------------|----------------------------------------------------------|---------|
@@ -28,6 +28,7 @@ All these metrics are prefixed with `etcd_server_`
 | proposals_applied_total   | The total number of consensus proposals applied.         | Gauge   |
 | proposals_pending         | The current number of pending proposals.                 | Gauge   |
 | proposals_failed_total    | The total number of failed proposals seen.               | Counter |
+| lease_expired_total       | The total number of expired leases.                      | Counter |
 
 `has_leader` indicates whether the member has a leader. If a member does not have a leader, it is
 totally unavailable. If all the members in the cluster do not have any leader, the entire cluster
@@ -43,6 +44,8 @@ is totally unavailable.
 
 `proposals_failed_total` are normally related to two issues: temporary failures related to a leader election or longer downtime caused by a loss of quorum in the cluster.
 
+`lease_expired_total` indicates how many leases have been expired. Rising expired leases correlates highly with rising proposals, since lease expiration happens via lease revoke proposals.
+
 ### Disk
 
 These metrics describe the status of the disk operations.