[MONIT-4105] values: add default value for prometheus server relabelings

In commit 2d80829 we added the support for custom prometheus server
relabelings.

This commit adds the default value for this option into the values.yaml
file.

Reported-at: https://its.cern.ch/jira/browse/MONIT-4105


Signed-off-by: Guillermo Facundo Colunga <guillermo.facundo.colunga@gmail.com>
Signed-off-by: Guillermo Facundo Colunga <guillermo.facundo.colunga@cern.ch>

values.yaml

@@ -118,6 +118,14 @@ metrics:
       #           app.kubernetes.io/instance: cern-magnum
       #           app.kubernetes.io/name: ingress-nginx
       serviceMonitors: []
+      # Allows to drop / relabel node Exporter metrics.
+      # More info on: https://prometheus.io/docs/prometheus/latest/configuration/configuration/#relabel_config
+      # Example:
+      # - action: drop
+      #  sourceLabels:
+      #  - __name__
+      #  regex: "my_custom_counter_total|my_custom_counter_sum|my_custom_gauge"
+      relabelings: []
   # This fluentbit is used to allow scraping and fordwarding metrics from
   # the local prometheus and send them to Open Telemetry Collector.
   # If the local .Values.metrics.prometheus enable=false will not be able