diff --git a/config/config-observability.yaml b/config/config-observability.yaml
index 6b002ab0d727..4d736ed12376 100644
--- a/config/config-observability.yaml
+++ b/config/config-observability.yaml
@@ -89,8 +89,8 @@ data:
     metrics.backend-destination: prometheus
 
     # metrics.request-metrics-backend-destination specifies the request metrics
-    # destination. If non-empty, it enables queue proxy to send request metrics.
-    # Currently supported values: prometheus, stackdriver.
+    # destination. It enables queue proxy to send request metrics.
+    # Currently supported values: prometheus (the default), stackdriver.
     metrics.request-metrics-backend-destination: prometheus
 
     # metrics.stackdriver-project-id field specifies the stackdriver project ID. This
diff --git a/pkg/metrics/config.go b/pkg/metrics/config.go
index 110240717ee7..8c847060aa9c 100644
--- a/pkg/metrics/config.go
+++ b/pkg/metrics/config.go
@@ -24,7 +24,8 @@ import (
 )
 
 const (
-	defaultLogURLTemplate = "http://localhost:8001/api/v1/namespaces/knative-monitoring/services/kibana-logging/proxy/app/kibana#/discover?_a=(query:(match:(kubernetes.labels.knative-dev%2FrevisionUID:(query:'${REVISION_UID}',type:phrase))))"
+	defaultLogURLTemplate        = "http://localhost:8001/api/v1/namespaces/knative-monitoring/services/kibana-logging/proxy/app/kibana#/discover?_a=(query:(match:(kubernetes.labels.knative-dev%2FrevisionUID:(query:'${REVISION_UID}',type:phrase))))"
+	defaultRequestMetricsBackend = "prometheus"
 )
 
 // ObservabilityConfig contains the configuration defined in the observability ConfigMap.
@@ -79,6 +80,8 @@ func NewObservabilityConfigFromConfigMap(configMap *corev1.ConfigMap) (*Observab
 
 	if mb, ok := configMap.Data["metrics.request-metrics-backend-destination"]; ok {
 		oc.RequestMetricsBackend = mb
+	} else {
+		oc.RequestMetricsBackend = defaultRequestMetricsBackend
 	}
 
 	if prof, ok := configMap.Data["profiling.enable"]; ok {
diff --git a/pkg/metrics/config_test.go b/pkg/metrics/config_test.go
index 7de4469e2f3f..b5318b40bc00 100644
--- a/pkg/metrics/config_test.go
+++ b/pkg/metrics/config_test.go
@@ -78,7 +78,7 @@ func TestObservabilityConfiguration(t *testing.T) {
 			EnableVarLogCollection: false,
 			LoggingURLTemplate:     defaultLogURLTemplate,
 			RequestLogTemplate:     "",
-			RequestMetricsBackend:  "",
+			RequestMetricsBackend:  "prometheus",
 		},
 		config: &corev1.ConfigMap{
 			ObjectMeta: metav1.ObjectMeta{