Add scaler metric latency support

CHANGELOG.md

@@ -48,6 +48,7 @@ To learn more about active deprecations, we recommend checking [GitHub Discussio
 
 Here is an overview of all **stable** additions:
 
+- **Prometheus Metrics**: Introduce scaler latency in Prometheus metrics. ([#4037](https://github.com/kedacore/keda/issues/4037))
 - **General**: Introduce new ArangoDB Scaler ([#4000](https://github.com/kedacore/keda/issues/4000))
 
 Here is an overview of all new **experimental** features:

pkg/prommetrics/prommetrics.go

@@ -55,6 +55,15 @@ var (
 		},
 		metricLabels,
 	)
+	scalerMetricsLatency = prometheus.NewGaugeVec(
+		prometheus.GaugeOpts{
+			Namespace: DefaultPromMetricsNamespace,
+			Subsystem: "scaler",
+			Name:      "metrics_latency",
+			Help:      "Scaler Metrics Latency",
+		},
+		metricLabels,
+	)
 	scalerErrors = prometheus.NewCounterVec(
 		prometheus.CounterOpts{
 			Namespace: DefaultPromMetricsNamespace,
@@ -96,6 +105,7 @@ var (
 func init() {
 	metrics.Registry.MustRegister(scalerErrorsTotal)
 	metrics.Registry.MustRegister(scalerMetricsValue)
+	metrics.Registry.MustRegister(scalerMetricsLatency)
 	metrics.Registry.MustRegister(scalerErrors)
 	metrics.Registry.MustRegister(scaledObjectErrors)
 
@@ -108,6 +118,11 @@ func RecordScalerMetric(namespace string, scaledObject string, scaler string, sc
 	scalerMetricsValue.With(getLabels(namespace, scaledObject, scaler, scalerIndex, metric)).Set(value)
 }
 
+// RecordScalerLatency create a measurement of the latency to external metric
+func RecordScalerLatency(namespace string, scaledObject string, scaler string, scalerIndex int, metric string, value float64) {
+	scalerMetricsLatency.With(getLabels(namespace, scaledObject, scaler, scalerIndex, metric)).Set(value)
+}
+
 // RecordScalerError counts the number of errors occurred in trying get an external metric used by the HPA
 func RecordScalerError(namespace string, scaledObject string, scaler string, scalerIndex int, metric string, err error) {
 	if err != nil {

pkg/scaling/scale_handler.go

@@ -440,7 +440,10 @@ func (h *scaleHandler) GetScaledObjectMetrics(ctx context.Context, scaledObjectN
 				}
 
 				if !metricsFoundInCache {
+					startTime := time.Now()
 					metrics, err = cache.GetMetricsForScaler(ctx, scalerIndex, metricName)
+					scalerLatency := time.Since(startTime).Milliseconds()
+					prommetrics.RecordScalerLatency(scaledObjectNamespace, scaledObject.Name, scalerName, scalerIndex, metricName, float64(scalerLatency))
 					h.logger.V(1).Info("Getting metrics from scaler", "scaledObject.Namespace", scaledObjectNamespace, "scaledObject.Name", scaledObjectName, "scaler", scalerName, "metricName", metricSpec.External.Metric.Name, "metrics", metrics, "scalerError", err)
 				}
 				metrics, err = fallback.GetMetricsWithFallback(ctx, h.client, h.logger, metrics, err, metricName, scaledObject, metricSpec)

tests/internals/prometheus_metrics/prometheus_metrics_test.go

@@ -21,6 +21,7 @@ import (
 
 const (
 	testName = "prometheus-metrics-test"
+	labelScaledObject = "scaledObject"
 )
 
 var (
@@ -225,6 +226,7 @@ func TestScaler(t *testing.T) {
 		"replica count should be 2 after 2 minute")
 
 	testScalerMetricValue(t)
+	testScalerMetricLatency(t)
 	testMetricsServerScalerMetricValue(t)
 	testOperatorMetrics(t, kc, data)
 
@@ -274,7 +276,7 @@ func testScalerMetricValue(t *testing.T) {
 		for _, metric := range metrics {
 			labels := metric.GetLabel()
 			for _, label := range labels {
-				if *label.Name == "scaledObject" && *label.Value == scaledObjectName {
+				if *label.Name == labelScaledObject && *label.Value == scaledObjectName {
 					assert.Equal(t, float64(4), *metric.Gauge.Value)
 					found = true
 				}
@@ -286,6 +288,29 @@ func testScalerMetricValue(t *testing.T) {
 	}
 }
 
+func testScalerMetricLatency(t *testing.T) {
+	t.Log("--- testing scaler metric latency ---")
+
+	family := fetchAndParsePrometheusMetrics(t, fmt.Sprintf("curl --insecure %s", kedaOperatorPrometheusURL))
+
+	if val, ok := family["keda_scaler_metrics_latency"]; ok {
+		var found bool
+		metrics := val.GetMetric()
+		for _, metric := range metrics {
+			labels := metric.GetLabel()
+			for _, label := range labels {
+				if *label.Name == labelScaledObject && *label.Value == scaledObjectName {
+					assert.Equal(t, float64(0), *metric.Gauge.Value)
+					found = true
+				}
+			}
+		}
+		assert.Equal(t, true, found)
+	} else {
+		t.Errorf("metric not available")
+	}
+}
+
 // [DEPRECATED] handle exporting Prometheus metrics from Operator to Metrics Server
 func testMetricsServerScalerMetricValue(t *testing.T) {
 	t.Log("--- testing scaler metric value in metrics server ---")
@@ -298,7 +323,7 @@ func testMetricsServerScalerMetricValue(t *testing.T) {
 		for _, metric := range metrics {
 			labels := metric.GetLabel()
 			for _, label := range labels {
-				if *label.Name == "scaledObject" && *label.Value == scaledObjectName {
+				if *label.Name == labelScaledObject && *label.Value == scaledObjectName {
 					assert.Equal(t, float64(4), *metric.Gauge.Value)
 					found = true
 				}