Allow partial metric updates

pkg/epp/backend/metrics/metrics.go

@@ -39,7 +39,8 @@ type PodMetricsClientImpl struct {
 	MetricMapping *MetricMapping
 }
 
-// FetchMetrics fetches metrics from a given pod.
+// FetchMetrics fetches metrics from a given pod, clones the existing metrics object and returns an
+// updated one.
 func (p *PodMetricsClientImpl) FetchMetrics(
 	ctx context.Context,
 	pod *Pod,

pkg/epp/backend/metrics/pod_metrics.go

@@ -116,16 +116,21 @@ func (pm *podMetrics) refreshMetrics() error {
 	updated, err := pm.pmc.FetchMetrics(ctx, pm.GetPod(), pm.GetMetrics(), pool.Spec.TargetPortNumber)
 	if err != nil {
 		pm.logger.V(logutil.TRACE).Info("Failed to refreshed metrics:", "err", err)
-		// As refresher is running in the background, it's possible that the pod is deleted but
-		// the refresh goroutine doesn't read the done channel yet. In this case, we just return nil.
-		// The refresher will be stopped after this interval.
-		return nil
 	}
-	updated.UpdateTime = time.Now()
-
-	pm.logger.V(logutil.TRACE).Info("Refreshed metrics", "updated", updated)
+	// Optimistically update metrics even if there was an error.
+	// The FetchMetrics can return an error for the following reasons:
+	// 1. As refresher is running in the background, it's possible that the pod is deleted but
+	// the refresh goroutine doesn't read the done channel yet. In this case, the updated
+	// metrics object will be nil. And the refresher will soon be stopped.
+	// 2. The FetchMetrics call can partially fail. For example, due to one metric missing. In
+	// this case, the updated metrics object will have partial updates. A partial update is
+	// considered better than no updates.
+	if updated != nil {
+		updated.UpdateTime = time.Now()
+		pm.logger.V(logutil.TRACE).Info("Refreshed metrics", "updated", updated)
+		atomic.StorePointer(&pm.metrics, unsafe.Pointer(updated))
+	}
 
-	atomic.StorePointer(&pm.metrics, unsafe.Pointer(updated))
 	return nil
 }