[prometheus-json-exporter] additionalMetricsRelabels: support full relabeling spec (regex, sourceLabels, actions)

[krishanthisera]

Is your feature request related to a problem ?

Yes

The additionalMetricsRelabels field in the prometheus-json-exporter ServiceMonitor template currently only supports simple key-value string replacements. This limits its capability compared to the full Prometheus relabeling specification.

See: here

Current Implementation
The current template hardcodes simple replacements:

{{- range $targetLabel, $replacement := .additionalMetricsRelabels | default $.Values.serviceMonitor.defaults.additionalMetricsRelabels }}
- targetLabel: {{ $targetLabel }}
  replacement: {{ $replacement }}
  action: replace
{{- end }}

This only allows simple label replacement:

additionalMetricsRelabels:
  label1: "value1"
  label2: "value2"

Expected Behaviour

The template should support the full Prometheus relabeling spec, allowing users to define:

additionalMetricsRelabels:
  - sourceLabels: [some_label]
    regex: '(.+)\.(.+)'
    replacement: '${1}-${2}'
    targetLabel: custom_label
    action: replace

---

Describe the solution you'd like.

Update the template to iterate over a list of relabeling configurations instead of a map:

...
    metricRelabelings:
     ...
      {{- with .additionalMetricsRelabels | default $.Values.serviceMonitor.defaults.additionalMetricsRelabels }}
      {{- toYaml . | nindent 6 }}
      {{- end }}

Describe alternatives you've considered.

N/A

Additional context.

No response