charts: Customize Prometheus time and size based retention

We set these with special variables that will be
transformed into Jinja template by the chart
renderer script which will allow to customize
these fields at runtime through CSC mechanisms.

Refs: #2953

charts/kube-prometheus-stack.yaml

@@ -118,6 +118,12 @@ prometheus:
             matchLabels:
               app.kubernetes.io/name: prometheus-operator-prometheus
 
+    ## How long to retain metrics
+    retention: '__var__(prometheus.spec.config.retention_time)'
+
+    ## Maximum size of metrics
+    retentionSize: '__var__(prometheus.spec.config.retention_size)'
+
 grafana:
   adminPassword: admin