I am trying to auto-scale my redis workers based on queue size, I am collecting the metrics using redis_exporter and promethues-to-sd sidecars in my redis deployment as so:

spec:
  containers:
    - name: master
      image: redis
      env:
        - name: MASTER
          value: "true"
      ports:
        - containerPort: 6379
      resources:
        limits:
          cpu: "100m"
        requests:
          cpu: "100m"
    - name: redis-exporter
      image: oliver006/redis_exporter:v0.21.1
      env:
      ports:
        - containerPort: 9121
      args: ["--check-keys=rq*"]
      resources:
        requests:
          cpu: 100m
          memory: 100Mi
    - name: prometheus-to-sd
      image: gcr.io/google-containers/prometheus-to-sd:v0.9.2
      command:
        - /monitor
        - --source=:http://localhost:9121
        - --stackdriver-prefix=custom.googleapis.com
        - --pod-id=$(POD_ID)
        - --namespace-id=$(POD_NAMESPACE)
        - --scrape-interval=15s
        - --export-interval=15s
      env:
        - name: POD_ID
          valueFrom:
            fieldRef:
              apiVersion: v1
              fieldPath: metadata.uid
        - name: POD_NAMESPACE
          valueFrom:
            fieldRef:
              fieldPath: metadata.namespace
      resources:
        requests:
          cpu: 100m
          memory: 100Mi

I can then view the metric (redis_key_size) in Metrics Explorer as:

metric.type="custom.googleapis.com/redis_key_size" 
resource.type="gke_container"

(I CAN'T view the metric if I change resource.type=k8_pod)

However I can't seem to get the HPA to read in these metrics getting a failed to get metrics error, and can't seem to figure out the correct Object definition.

I've tried both .object.target.kind=Pod and Deployment, with deployment I get the additional error "Get namespaced metric by name for resource \"deployments\"" is not implemented.

I don't know if this issue is related to the resource.type="gke_container" and how to change that?

apiVersion: autoscaling/v2beta1
kind: HorizontalPodAutoscaler
metadata:
  name: {{ template "webapp.backend.fullname" . }}-workers
  namespace: default
spec:
  scaleTargetRef:
    apiVersion: apps/v1
    kind: Deployment
    name: {{ template "webapp.backend.fullname" . }}-workers
  minReplicas: 1
  maxReplicas: 4
  metrics:
    - type: Object
      object:
        target:
          kind: <not sure>
          name: <not sure>
        metricName: redis_key_size
        targetValue: 4

--- Update ---

This works if I use kind: Pod and manually set name to the pod name created by the deployment, however this is far from perfect.

I also tried this setup using type Pods, however the HPA says it can't read the metrics horizontal-pod-autoscaler failed to get object metric value: unable to get metric redis_key_size: no metrics returned from custom metrics API

apiVersion: autoscaling/v2beta1
kind: HorizontalPodAutoscaler
metadata:
  name: {{ template "webapp.backend.fullname" . }}-workers
  namespace: default
spec:
  scaleTargetRef:
    apiVersion: apps/v1
    kind: Deployment
    name: {{ template "webapp.backend.fullname" . }}-workers
  minReplicas: 1
  maxReplicas: 4
  metrics:
  - type: Pods
    pods:
      metricName: redis_key_size
      targetAverageValue: 4