etcdserver: Improve some debug metrics.

The _total suffix is by convention for counters,
don't use it on a gauge. Clarify help string.
Tweak metric name so it'll sort with related metrics,
and be a little more understandable.

Remove open file descriptor metric, as Prometheus client_golang
provides that out of the box as process_open_fds which is also
more up to date. Both only support Linux, so there's no loss of
platform support.

Fixes #5229

Documentation/metrics.md

@@ -79,16 +79,13 @@ The metrics under the `etcd_debugging` prefix are for debugging. They are very i
 
 | Name                                    | Description                                      | Type      |
 |-----------------------------------------|--------------------------------------------------|-----------|
-| file_descriptors_used_total             | The total number of file descriptors used        | Gauge     |
 | proposal_durations_seconds              | The latency distributions of committing proposal | Histogram |
-| pending_proposal_total                  | The total number of pending proposals            | Gauge     |
+| proposals_pending                       | The current number of pending proposals          | Gauge     |
 | proposal_failed_total                   | The total number of failed proposals             | Counter   |
 
-Heavy file descriptor (`file_descriptors_used_total`) usage (i.e., near the process's file descriptor limit) indicates a potential file descriptor exhaustion issue. If the file descriptors are exhausted, etcd may panic because it cannot create new WAL files.
-
 [Proposal][glossary-proposal] durations (`proposal_durations_seconds`) provides a proposal commit latency histogram. The reported latency reflects network and disk IO delays in etcd.
 
-Pending proposal (`pending_proposal_total`) indicates how many proposals are queued for commit. A rising pending proposal total suggests there is a high client load or the cluster is unstable.
+Proposals pending (`proposals_pending`) indicates how many proposals are queued for commit. Rising pending proposals suggests there is a high client load or the cluster is unstable.
 
 Failed proposals (`proposal_failed_total`) are normally related to two issues: temporary failures related to a leader election or longer duration downtime caused by a loss of quorum in the cluster.
 
@@ -127,6 +124,17 @@ Label `msgType` is the type of raft message. `MsgApp` is log replication message
 
 Label `remoteID` is the member ID of the message destination.
 
+## Prometheus supplied metrics
+
+The Prometheus client library provides a number of metrics under the `go` and `process` namespaces. There are a few that are particlarly interesting.
+
+| Name                              | Description                                | Type         |
+|-----------------------------------|--------------------------------------------|--------------|
+| process_open_fds                  | Number of open file descriptors.           | Gauge        |
+| process_max_fds                   | Maximum number of open file descriptors.   | Gauge        |
+
+Heavy file descriptor (`process_open_fds`) usage (i.e., near the process's file descriptor limit, `process_max_fds`) indicates a potential file descriptor exhaustion issue. If the file descriptors are exhausted, etcd may panic because it cannot create new WAL files.
+
 [glossary-proposal]: glossary.md#proposal
 [prometheus]: http://prometheus.io/
 [prometheus-getting-started]: http://prometheus.io/docs/introduction/getting_started/

etcdserver/metrics.go

@@ -33,8 +33,8 @@ var (
 	proposePending = prometheus.NewGauge(prometheus.GaugeOpts{
 		Namespace: "etcd_debugging",
 		Subsystem: "server",
-		Name:      "pending_proposal_total",
-		Help:      "The total number of pending proposals.",
+		Name:      "proposals_pending",
+		Help:      "The current number of pending proposals.",
 	})
 	// This is number of proposal failed in client's view.
 	// The proposal might be later got committed in raft.
@@ -44,20 +44,12 @@ var (
 		Name:      "proposal_failed_total",
 		Help:      "The total number of failed proposals.",
 	})
-
-	fileDescriptorUsed = prometheus.NewGauge(prometheus.GaugeOpts{
-		Namespace: "etcd_debugging",
-		Subsystem: "server",
-		Name:      "file_descriptors_used_total",
-		Help:      "The total number of file descriptors used.",
-	})
 )
 
 func init() {
 	prometheus.MustRegister(proposeDurations)
 	prometheus.MustRegister(proposePending)
 	prometheus.MustRegister(proposeFailed)
-	prometheus.MustRegister(fileDescriptorUsed)
 }
 
 func monitorFileDescriptor(done <-chan struct{}) {
@@ -69,7 +61,6 @@ func monitorFileDescriptor(done <-chan struct{}) {
 			plog.Errorf("cannot monitor file descriptor usage (%v)", err)
 			return
 		}
-		fileDescriptorUsed.Set(float64(used))
 		limit, err := runtime.FDLimit()
 		if err != nil {
 			plog.Errorf("cannot monitor file descriptor usage (%v)", err)