I'm trying to scale a Kubernetes Deployment using a HorizontalPodAutoscaler, which listens to a custom metrics through Stackdriver.

I'm having a GKE cluster, with a Stackdriver adapter enabled. I'm able to publish the custom metric type to Stackdriver, and following is the way it's being displayed in Stackdriver's Metric Explorer.

This is how I have defined my HPA:

apiVersion: autoscaling/v2beta1
kind: HorizontalPodAutoscaler
metadata:
  name: example-hpa
spec:
  minReplicas: 1
  maxReplicas: 10
  metrics:
  - type: External
    external:
      metricName: custom.googleapis.com|worker_pod_metrics|baz
      targetValue: 400
  scaleTargetRef:
    apiVersion: apps/v1
    kind: Deployment
    name: test-app-group-1-1

After successfully creating example-hpa, executing kubectl get hpa example-hpa, always shows TARGETS as <unknown>, and never detects the value from custom metrics.

NAME          REFERENCE                       TARGETS         MINPODS   MAXPODS   REPLICAS   AGE
example-hpa   Deployment/test-app-group-1-1   <unknown>/400   1         10        1          18m

I'm using a Java client which runs locally to publish my custom metrics. I have given the appropriate resource labels as mentioned here (hard coded - so that it can run without a problem in local environment). I have followed this document to create the Java client.

private static MonitoredResource prepareMonitoredResourceDescriptor() {
        Map<String, String> resourceLabels = new HashMap<>();
        resourceLabels.put("project_id", "<<<my-project-id>>>);
        resourceLabels.put("pod_id", "<my pod UID>");
        resourceLabels.put("container_name", "");
        resourceLabels.put("zone", "asia-southeast1-b");
        resourceLabels.put("cluster_name", "my-cluster");
        resourceLabels.put("namespace_id", "mynamespace");
        resourceLabels.put("instance_id", "");

        return MonitoredResource.newBuilder()
                .setType("gke_container")
                .putAllLabels(resourceLabels)
                .build();
    }

What am I doing wrong in the above-mentioned steps please? Thank you in advance for any answers provided!

EDIT [RESOLVED]: I think I have had some misconfigurations, since kubectl describe hpa [NAME] --v=9 showed me some 403 status code, as well as I was using type: External instead of type: Pods (Thanks MWZ for your answer, pointing out this mistake).

I managed to fix it by creating a new project, a new service account, and a new GKE cluster (basically everything from the beginning again). Then I changed my yaml file as follows, exactly as this document explains.

apiVersion: autoscaling/v2beta1
kind: HorizontalPodAutoscaler
metadata:
  name: test-app-group-1-1
  namespace: default
spec:
  scaleTargetRef:
    apiVersion: apps/v1beta1
    kind: Deployment
    name: test-app-group-1-1
  minReplicas: 1
  maxReplicas: 5
  metrics:
  - type: Pods                 # Earlier this was type: External
    pods:                      # Earlier this was external:
      metricName: baz                               # metricName: custom.googleapis.com|worker_pod_metrics|baz
      targetAverageValue: 20

I'm now exporting as custom.googleapis.com/baz, and NOT as custom.googleapis.com/worker_pod_metrics/baz. Also, now I'm explicitly specifying the namespace for my HPA in the yaml.