Added rest client metrics for client TTL and rot. (#84382)

* Added rest client metrics for client TTL and rot.

* Fixed foo bar comment, added nil checks

* Moved rotation observation inside of old cert nil check block

* Fixed rotation age logic.

* fixed BUILD for exec plugin package

* fixed null pointer dereference in exec.go

* Updated metric name, bucket, used oldest cert.

Kubernetes-commit: 9dcb3bfcff2024e4690d70262cad14668b1f2507

tools/metrics/metrics.go

@@ -26,6 +26,16 @@ import (
 
 var registerMetrics sync.Once
 
+// DurationMetric is a measurement of some amount of time.
+type DurationMetric interface {
+	Observe(duration time.Duration)
+}
+
+// TTLMetric sets the time to live of something.
+type TTLMetric interface {
+	Set(ttl *time.Duration)
+}
+
 // LatencyMetric observes client latency partitioned by verb and url.
 type LatencyMetric interface {
 	Observe(verb string, u url.URL, latency time.Duration)
@@ -37,21 +47,51 @@ type ResultMetric interface {
 }
 
 var (
+	// ClientCertTTL is the time to live of a client certificate
+	ClientCertTTL TTLMetric = noopTTL{}
+	// ClientCertRotationAge is the age of a certificate that has just been rotated.
+	ClientCertRotationAge DurationMetric = noopDuration{}
 	// RequestLatency is the latency metric that rest clients will update.
 	RequestLatency LatencyMetric = noopLatency{}
 	// RequestResult is the result metric that rest clients will update.
 	RequestResult ResultMetric = noopResult{}
 )
 
+// RegisterOpts contains all the metrics to register. Metrics may be nil.
+type RegisterOpts struct {
+	ClientCertTTL         TTLMetric
+	ClientCertRotationAge DurationMetric
+	RequestLatency        LatencyMetric
+	RequestResult         ResultMetric
+}
+
 // Register registers metrics for the rest client to use. This can
 // only be called once.
-func Register(lm LatencyMetric, rm ResultMetric) {
+func Register(opts RegisterOpts) {
 	registerMetrics.Do(func() {
-		RequestLatency = lm
-		RequestResult = rm
+		if opts.ClientCertTTL != nil {
+			ClientCertTTL = opts.ClientCertTTL
+		}
+		if opts.ClientCertRotationAge != nil {
+			ClientCertRotationAge = opts.ClientCertRotationAge
+		}
+		if opts.RequestLatency != nil {
+			RequestLatency = opts.RequestLatency
+		}
+		if opts.RequestResult != nil {
+			RequestResult = opts.RequestResult
+		}
 	})
 }
 
+type noopDuration struct{}
+
+func (noopDuration) Observe(time.Duration) {}
+
+type noopTTL struct{}
+
+func (noopTTL) Set(*time.Duration) {}
+
 type noopLatency struct{}
 
 func (noopLatency) Observe(string, url.URL, time.Duration) {}