Merge PR #23: K8ssandra native Prometheus telemetry (Simone Maiorani)

Adds an optional telemetry.prometheus block to the K8ssandraCluster CR,
gated by .Values.cassandra.telemetry. When enabled, K8ssandra emits a
ServiceMonitor for Cassandra metrics that any cluster-side Prometheus
operator can scrape natively, with no separate ServiceMonitor manifest
to maintain.

The default values block + configuration.md documentation are added in
the follow-up commit.

Co-authored-by: Simone Maiorani <simone.maiorani@e4company.com>

Author: fbeneventi

deploy/helm/examon/templates/k8ssandra-cluster.yaml

@@ -36,6 +36,15 @@ spec:
         racks:
           {{- toYaml .Values.cassandra.datacenters.dc1.racks | nindent 10 }}
         {{- end }}
+        {{- with .Values.cassandra.telemetry }}
+        telemetry:
+          prometheus:
+            enabled: {{ .prometheus.enabled | default false }}
+            {{- with .prometheus.commonLabels }}
+            commonLabels:
+              {{- toYaml . | nindent 14 }}
+            {{- end }}
+        {{- end }}
   {{- if .Values.cassandra.reaper }}
   reaper:
     autoScheduling: