Introduce the TriggerTimeTracker util.

It will be used by the Endpoints Controller to compute and export the EndpointsLastChangeTriggerTime annotation, which in turn will be used to compute the In-Cluster Network Programming Latency SLI.
See https://github.com/kubernetes/community/blob/master/sig-scalability/slos/network_programming_latency.md for more details.

pkg/controller/endpoint/endpoints_controller.go

@@ -101,6 +101,8 @@ func NewEndpointController(podInformer coreinformers.PodInformer, serviceInforme
 	e.endpointsLister = endpointsInformer.Lister()
 	e.endpointsSynced = endpointsInformer.Informer().HasSynced
 
+	e.triggerTimeTracker = NewTriggerTimeTracker()
+
 	return e
 }
 
@@ -138,6 +140,10 @@ type EndpointController struct {
 
 	// workerLoopPeriod is the time between worker runs. The workers process the queue of service and pod changes.
 	workerLoopPeriod time.Duration
+
+	// triggerTimeTracker is an util used to compute and export the EndpointsLastChangeTriggerTime
+	// annotation.
+	triggerTimeTracker *TriggerTimeTracker
 }
 
 // Run will not return until stopCh is closed. workers determines how many
@@ -399,6 +405,7 @@ func (e *EndpointController) syncService(key string) error {
 		if err != nil && !errors.IsNotFound(err) {
 			return err
 		}
+		e.triggerTimeTracker.DeleteEndpoints(namespace, name)
 		return nil
 	}
 
@@ -427,6 +434,12 @@ func (e *EndpointController) syncService(key string) error {
 		}
 	}
 
+	// We call ComputeEndpointsLastChangeTriggerTime here to make sure that the state of the trigger
+	// time tracker gets updated even if the sync turns out to be no-op and we don't update the
+	// endpoints object.
+	endpointsLastChangeTriggerTime := e.triggerTimeTracker.
+		ComputeEndpointsLastChangeTriggerTime(namespace, name, service, pods)
+
 	subsets := []v1.EndpointSubset{}
 	var totalReadyEps int
 	var totalNotReadyEps int
@@ -506,6 +519,11 @@ func (e *EndpointController) syncService(key string) error {
 		newEndpoints.Annotations = make(map[string]string)
 	}
 
+	if !endpointsLastChangeTriggerTime.IsZero() {
+		newEndpoints.Annotations[v1.EndpointsLastChangeTriggerTime] =
+			endpointsLastChangeTriggerTime.Format(time.RFC3339Nano)
+	}
+
 	klog.V(4).Infof("Update endpoints for %v/%v, ready: %d not ready: %d", service.Namespace, service.Name, totalReadyEps, totalNotReadyEps)
 	if createEndpoints {
 		// No previous endpoints, create them