[helm] Add support for relabelings and metricRelabelings at serviceMonitor

[jkroepke]

Description

I need to add some additional labels to all metrics.

Fixes #ISSUE

Checklist

• Unit tests updated
• End user documentation updated

[jkroepke]

@stevehipwell I would appreciate any feedback here.

[stevehipwell]

I think it might be best to reduce the tight coupling with the ServiceMonitor spec and provide a more flexible solution where we don't need to add all the fields into the values?

serviceMonitor:
  enabled: false
  namespace:
  additionalLabels: {}
  annotations: {}
  targetLabels: []
  endpointConfig:
    interval: 1m
    scrapeTimeout: 10s

Then if you wanted to template this you could use a string as so.

serviceMonitor:
  enabled: false
  namespace:
  additionalLabels: {}
  annotations: {}
  targetLabels: []
  endpointConfig: |-
    interval: 1m
    scrapeTimeout: 10s
    metricRelabelings:
      {{ .Values.test }}

[jkroepke]

I remove the tpl function. I do not have a use case for it rn, I copied it from prometheus helm chart.

I can implement the ServiceMonitor flexible approach with the endpointConfig. But it would be a really special way compared to other world known helm charts. So please re-confirm we if you want go the more generic approch.

I personally would prefer to follow the same pattern as other helm charts are doing this to keep a consistent chart experience

[stevehipwell]

@jkroepke I think the problem here is twofold. A lot of the well known Helm charts don't have much of a submission gate which results in completely incorrect implementations such as tpl for an object and API designs which have grown organically rather than there having been though applied. This leads to a naïve implementation requiring the duplication of a resource spec in both the values and the template leading to a brittle chart. For my charts I use the endpointConfig pattern to save this issue, but I get that the other pattern has become pervasive. Let's keep your implementation for now and I'll look at using an omit pattern to remove the explicit tight coupling in the template.

[stevehipwell]

/lgtm

[jkroepke]

Thanks @stevehipwell , I'm looking forward on this PR

[stevehipwell]

@Raffo could you approve this or do you want me to LGTM & approve these myself?

[Raffo]

@stevehipwell I can approve this one, but I have zero problem with you doing it. Feel free to own help related changes.

[Raffo]

/approve

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