Paper prototype on how HPA with custom KPA scaler may look like

hpa-kpa.patch

--- a
+++ b
-apiVersion: autoscaling/v2
+apiVersion: autoscaling/v3
kind: HorizontalPodAutoscaler
metadata:
  name: php-apache
spec:
  scaleTargetRef:
    apiVersion: apps/v1
    kind: Deployment
    name: php-apache
  minReplicas: 1
  maxReplicas: 10
+ algorithm: custom://scaler-api.kpa.svc
  metrics:
  - type: Resource
    resource:
      name: cpu
      target:
        type: Utilization
        averageUtilization: 50
+ - type: KPA
+   kpa:
+     name: requests_per_second
+     target:
+       type: Average
+       value: 100