fix: metric lag calculation for update events

the lastUpdated time was calculated against the informer's registration
time instead of the current time.

scrapers/kubernetes/informers.go

@@ -87,7 +87,7 @@ type SharedInformerManager struct {
 type DeleteObjHandler func(ctx context.Context, id string) error
 
 func (t *SharedInformerManager) Register(ctx api.ScrapeContext, watchResource v1.KubernetesResourceToWatch, queue *pq.Queue) error {
-	start := time.Now()
+	registrationTime := time.Now()
 
 	apiVersion, kind := watchResource.ApiVersion, watchResource.Kind
 
@@ -127,11 +127,11 @@ func (t *SharedInformerManager) Register(ctx api.ScrapeContext, watchResource v1
 				"type", "add",
 				"kind", u.GetKind(),
 				"scraper_id", ctx.ScraperID(),
-				"valid_timestamp", lo.Ternary(u.GetCreationTimestamp().Time.After(start), "true", "false"),
+				"valid_timestamp", lo.Ternary(u.GetCreationTimestamp().Time.After(registrationTime), "true", "false"),
 			).Add(1)
 
 			// This is a way to avoid instrumenting old objects so they don't skew the lag time.
-			if u.GetCreationTimestamp().Time.After(start) {
+			if u.GetCreationTimestamp().Time.After(registrationTime) {
 				ctx.Histogram("informer_receive_lag", informerLagBuckets,
 					"scraper", ctx.ScraperID(),
 					"kind", watchResource.Kind,
@@ -140,6 +140,11 @@ func (t *SharedInformerManager) Register(ctx api.ScrapeContext, watchResource v1
 			}
 		},
 		UpdateFunc: func(oldObj any, newObj any) {
+			// Because the kubernetes object timestamp are precise only upto a second.
+			// we need to round down the current time to the second.
+			// Else the current time will be just a few milliseconds ahead and we the last updated time in the past.
+			now := time.Now().UTC().Truncate(time.Second)
+
 			u, err := getUnstructuredFromInformedObj(watchResource, newObj)
 			if err != nil {
 				ctx.Counter("kubernetes_informer_errors",
@@ -156,23 +161,23 @@ func (t *SharedInformerManager) Register(ctx api.ScrapeContext, watchResource v1
 			}
 
 			lastUpdatedTime := health.GetLastUpdatedTime(u)
-			lastUpdatedInPast := lastUpdatedTime != nil && lastUpdatedTime.After(u.GetCreationTimestamp().Time) && lastUpdatedTime.Before(start)
-			if lastUpdatedInPast {
+			lastUpdatedInFuture := lastUpdatedTime != nil && lastUpdatedTime.After(now)
+			if !lastUpdatedInFuture {
 				ctx.Histogram("informer_receive_lag", informerLagBuckets,
 					"scraper", ctx.ScraperID(),
 					"kind", watchResource.Kind,
 					"operation", "update",
-				).Record(time.Duration(time.Since(*lastUpdatedTime).Milliseconds()))
+				).Record(time.Duration(now.Sub(*lastUpdatedTime).Milliseconds()))
 			} else {
 				ctx.Warnf("%s/%s/%s has last updated time %s into the future. now=%s, lastupdatedTime=%s",
-					u.GetKind(), u.GetNamespace(), u.GetName(), time.Until(*lastUpdatedTime), start, *lastUpdatedTime)
+					u.GetKind(), u.GetNamespace(), u.GetName(), lastUpdatedTime.Sub(now), now, *lastUpdatedTime)
 			}
 
 			ctx.Counter("kubernetes_informer_events",
 				"type", "update",
 				"kind", u.GetKind(),
 				"scraper_id", ctx.ScraperID(),
-				"valid_timestamp", lo.Ternary(lastUpdatedInPast, "true", "false"),
+				"valid_timestamp", lo.Ternary(!lastUpdatedInFuture, "true", "false"),
 			).Add(1)
 
 			queue.Enqueue(NewQueueItem(u, QueueItemOperationUpdate))