stable/prometheus: restore headless svcs, reverting breaking change to the default values (#5529)

This commit reverts a breaking change that was introduced in the
following PR: https://github.com/kubernetes/charts/pull/4915

The problems caused by this change are described in the following GitHub
issue: https://github.com/kubernetes/charts/issues/5128

The root of the problem is that the `clusterIP` field on services is not
mutable, so a simple helm upgrade is not sufficient to handle this type
of change. Furthermore, since these are overridable values, users who
have a specific use case for making the services non-headless can do so
in their overrides for the chart installation.

Closes https://github.com/kubernetes/charts/issues/5128

stable/prometheus/Chart.yaml

@@ -1,5 +1,5 @@
 name: prometheus
-version: 6.5.1
+version: 6.5.2
 appVersion: 2.2.1
 description: Prometheus is a monitoring system and time series database.
 home: https://prometheus.io/

stable/prometheus/values.yaml

@@ -316,7 +316,9 @@ kubeStateMetrics:
       prometheus.io/scrape: "true"
     labels: {}
 
-    clusterIP: ""
+    # Exposed as a headless service:
+    # https://kubernetes.io/docs/concepts/services-networking/service/#headless-services
+    clusterIP: None
 
     ## List of IP addresses at which the kube-state-metrics service is available
     ## Ref: https://kubernetes.io/docs/user-guide/services/#external-ips
@@ -406,7 +408,9 @@ nodeExporter:
       prometheus.io/scrape: "true"
     labels: {}
 
-    clusterIP: ""
+    # Exposed as a headless service:
+    # https://kubernetes.io/docs/concepts/services-networking/service/#headless-services
+    clusterIP: None
 
     ## List of IP addresses at which the node-exporter service is available
     ## Ref: https://kubernetes.io/docs/user-guide/services/#external-ips