exec credential provider: add rest_client_exec_plugin_call_total metric

Signed-off-by: Andrew Keesler <akeesler@vmware.com>

Kubernetes-commit: 31eec29b098f790cd96fd6d2441e68938f15363b

tools/metrics/metrics.go

@@ -46,6 +46,12 @@ type ResultMetric interface {
 	Increment(code string, method string, host string)
 }
 
+// CallsMetric counts calls that take place for a specific exec plugin.
+type CallsMetric interface {
+	// Increment increments a counter per exitCode and callStatus.
+	Increment(exitCode int, callStatus string)
+}
+
 var (
 	// ClientCertExpiry is the expiry time of a client certificate
 	ClientCertExpiry ExpiryMetric = noopExpiry{}
@@ -57,6 +63,9 @@ var (
 	RateLimiterLatency LatencyMetric = noopLatency{}
 	// RequestResult is the result metric that rest clients will update.
 	RequestResult ResultMetric = noopResult{}
+	// ExecPluginCalls is the number of calls made to an exec plugin, partitioned by
+	// exit code and call status.
+	ExecPluginCalls CallsMetric = noopCalls{}
 )
 
 // RegisterOpts contains all the metrics to register. Metrics may be nil.
@@ -66,6 +75,7 @@ type RegisterOpts struct {
 	RequestLatency        LatencyMetric
 	RateLimiterLatency    LatencyMetric
 	RequestResult         ResultMetric
+	ExecPluginCalls       CallsMetric
 }
 
 // Register registers metrics for the rest client to use. This can
@@ -87,6 +97,9 @@ func Register(opts RegisterOpts) {
 		if opts.RequestResult != nil {
 			RequestResult = opts.RequestResult
 		}
+		if opts.ExecPluginCalls != nil {
+			ExecPluginCalls = opts.ExecPluginCalls
+		}
 	})
 }
 
@@ -105,3 +118,7 @@ func (noopLatency) Observe(string, url.URL, time.Duration) {}
 type noopResult struct{}
 
 func (noopResult) Increment(string, string, string) {}
+
+type noopCalls struct{}
+
+func (noopCalls) Increment(int, string) {}