[Bug]: duplicated http in rest metrics fetcher (#408) (#421)

* bug fix: Duplicate 'http://' on calling RestMetricsFetcher.FetchPodMetrics (#408), Add abstractMetricsFetcher to make CustomMetricsFetcher, ResourceMetricsFetcher, and KubernetesMetricsFetcher comply MetricsFetcher.

* bug fix

---------

Co-authored-by: Jingyuan Zhang <jingyuan.zhang0929@bytedance.com>

pkg/controller/podautoscaler/metrics/fetcher.go

@@ -50,18 +50,23 @@ type MetricFetcher interface {
 	FetchMetric(ctx context.Context, endpoint string, path string, metricName string) (float64, error)
 }
 
+type abstractMetricsFetcher struct{}
+
+func (f *abstractMetricsFetcher) FetchMetric(ctx context.Context, pod v1.Pod, metricsPort int, metricName string) (float64, error) {
+	return 0.0, fmt.Errorf("not implemented")
+}
+
 // RestMetricsFetcher implements MetricFetcher to fetch metrics from Pod's /metrics endpoint.
 type RestMetricsFetcher struct{}
 
 func (f *RestMetricsFetcher) FetchPodMetrics(ctx context.Context, pod v1.Pod, metricsPort int, metricName string) (float64, error) {
-	// Use http to fetch pod's /metrics endpoint and parse the value
-	return f.FetchMetric(ctx, fmt.Sprintf("http://%s:%d", pod.Status.PodIP, metricsPort), "/metrics", metricName)
+	// Use /metrics to fetch pod's endpoint
+	return f.FetchMetric(ctx, fmt.Sprintf("%s:%d", pod.Status.PodIP, metricsPort), "/metrics", metricName)
 }
 
 func (f *RestMetricsFetcher) FetchMetric(ctx context.Context, endpoint string, path string, metricName string) (float64, error) {
-
-	// We should use the primary container port. In future, we can decide whether to use sidecar container's port
-	url := fmt.Sprintf("http://%s/%s", strings.TrimRight(endpoint, "/"), strings.TrimLeft(path, "/"))
+	// Use http to fetch endpoint
+	url := fmt.Sprintf("http://%s/%s", endpoint, strings.TrimLeft(path, "/"))
 
 	// Create request with context, so that the request will be canceled if the context is canceled
 	req, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
@@ -97,6 +102,7 @@ func (f *RestMetricsFetcher) FetchMetric(ctx context.Context, endpoint string, p
 
 // ResourceMetricsFetcher fetches resource metrics from Kubernetes metrics API (metrics.k8s.io).
 type ResourceMetricsFetcher struct {
+	abstractMetricsFetcher
 	metricsClient *versioned.Clientset
 }
 
@@ -124,6 +130,7 @@ func (f *ResourceMetricsFetcher) FetchPodMetrics(ctx context.Context, pod v1.Pod
 
 // CustomMetricsFetcher fetches custom metrics from Kubernetes' native Custom Metrics API.
 type CustomMetricsFetcher struct {
+	abstractMetricsFetcher
 	customMetricsClient custom_metrics.CustomMetricsClient
 }
 
@@ -157,6 +164,7 @@ func (f *CustomMetricsFetcher) FetchPodMetrics(ctx context.Context, pod v1.Pod,
 }
 
 type KubernetesMetricsFetcher struct {
+	abstractMetricsFetcher
 	resourceFetcher *ResourceMetricsFetcher
 	customFetcher   *CustomMetricsFetcher
 }