[MONIT-4105] prometheus/server: allow user to all relabelings

As reported by some users it would be very interesting for them to be
able to filter out metrics or transform them. Not all of them are
interested in keeping all the metrics that come by default with the
chart. Also they would like sometimes to rename some of the metrics or
labels that came out of the box.

Given prometheus architecture this should have been done on each target
scrape policy. Instead, given our current set up this can be centralised
in the remote write to the local fluent bit, applying the transformation
just before we push the metrics to the fluentbit and this fluentbit to
the monitoring infrastructure.

This commit modifies the prometheus server template in such a way that
if the user adds some custom relablings in the values they are appended
under the normal relabelings that are applied to every installation.
If no config is provided then nothing is modified.

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>

templates/prometheus/prometheus.yaml

@@ -49,6 +49,9 @@ spec:
           action: drop
         - regex: '(id|uuid)'
           action: labeldrop
+        {{- if .Values.metrics.prometheus.server.relabelings}}
+          {{- .Values.metrics.prometheus.server.relabelings | toYaml | nindent 8}}
+        {{- end }}
       metadataConfig:
         send: false
     {{if .Values.metrics.prometheus.server.remoteWrite.endpoint }}