feat(k8s): add a horizontal pod autoscaler

this scales up when the average CPU utilization is at 95% of the
requested CPU.

We should remove the replicas key from the deployment, otherwise any time
we deploy it will overwrite the HPA (source:
https://kubernetes.io/docs/tasks/run-application/horizontal-pod-autoscale/#migrating-deployments-and-statefulsets-to-horizontal-autoscaling)

Behaviour:

- every 15 seconds - The HPA controller checks the value of the metric used every 15 seconds per pod.
- every 3 minutes - The HPA scales up pods if the metric threshold has been continually exceeded for 3 mins.
- every 5 minutes - The HPA scales down pods if the metric threshold has not been exceeded for 5 mins.

Further info:

- https://user-guide.cloud-platform.service.justice.gov.uk/documentation/concepts/deploying.html#horizontal-pod-autoscaling-hpa
- https://kubernetes.io/docs/tasks/run-application/horizontal-pod-autoscale

.github/workflows/reusable-push-and-deploy.yml

@@ -118,6 +118,7 @@ jobs:
 
         run: |
           cat deployments/templates/deployment.yml | envsubst > deployments/deployment.yml
+          cat deployments/templates/hpa.yml | envsubst > deployments/hpa.yml
           cat deployments/templates/ingress.yml | envsubst > deployments/ingress.yml
           cat deployments/templates/service.yml | envsubst > deployments/service.yml
           cat deployments/templates/secrets.yml | envsubst > deployments/secrets.yml

deployments/templates/deployment.yml

@@ -5,7 +5,6 @@ metadata:
   labels:
     app: find-moj-data
 spec:
-  replicas: 2
   strategy:
     type: RollingUpdate
     rollingUpdate:

deployments/templates/hpa.yml

@@ -0,0 +1,19 @@
+apiVersion: autoscaling/v2
+kind: HorizontalPodAutoscaler
+metadata:
+  name: find-moj-data
+  namespace: ${NAMESPACE}
+spec:
+  scaleTargetRef:
+    apiVersion: apps/v1
+    kind: Deployment
+    name: find-moj-data-deployment
+  minReplicas: 1
+  maxReplicas: 5
+  metrics:
+    - type: Resource
+      resource:
+        name: cpu
+        target:
+          type: Utilization
+          averageUtilization: 95