[prometheus-blackbox-exporter] define metricRelabelings in selfservicemonitor.yaml to handle additionalMetricsRelabels

[Sheikh-Abubaker]

What this PR does / why we need it

Which issue this PR fixes

(optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged)

• fixes [prometheus-blackbox-exporter] additionalMetricsRelabels in the values file, but not handled in selfservicemonitor #4296

Special notes for your reviewer

Generated ServiceMonitor resource after defining metricRelabelings in selfservicemonitor.yaml, and when additionalMetricsRelabels set to an empty list.

helm template . -f override-values.yaml

---
# Source: prometheus-blackbox-exporter/templates/selfservicemonitor.yaml
apiVersion: monitoring.coreos.com/v1
kind: ServiceMonitor
metadata:
  name: release-name-prometheus-blackbox-exporter
  namespace: default
  labels:
    helm.sh/chart: prometheus-blackbox-exporter-8.13.0
    app.kubernetes.io/name: prometheus-blackbox-exporter
    app.kubernetes.io/instance: release-name
    app.kubernetes.io/version: "v0.24.0"
    app.kubernetes.io/managed-by: Helm
spec:
  endpoints:
  - path: /metrics
    interval: 30s
    scrapeTimeout: 30s
    scheme: http
    metricRelabelings:
      - action: keep
        regex: prometheus
        sourceLabels:
        - __meta_kubernetes_pod_label_team
      - action: drop
        regex: west_europe
        sourceLabels:
        - __meta_kubernetes_pod_label_datacenter
  jobLabel: "release-name"
  selector:
    matchLabels:
      app.kubernetes.io/name: prometheus-blackbox-exporter
      app.kubernetes.io/instance: release-name
  namespaceSelector:
    matchNames:
      - default

additionalMetricsRelabels changed to empty list due to this warning

helm template . -f override-values.yaml
 coalesce.go:286: warning: cannot overwrite table with non table for prometheus-blackbox-exporter.serviceMonitor.selfMonitor.additionalMetricsRelabels (map[])

# override-values.yaml 

serviceMonitor:
## If true, a ServiceMonitor CRD is created for a prometheus operator
## https://github.com/coreos/prometheus-operator for blackbox-exporter itself
##
selfMonitor:
  enabled: true
  additionalMetricsRelabels:
  - sourceLabels:
    - __meta_kubernetes_pod_label_team
    regex: "prometheus"
    action: keep
  - sourceLabels:
    - __meta_kubernetes_pod_label_datacenter
    regex: west_europe
    action: drop
  additionalRelabeling: []
  labels: {}
  path: /metrics
  interval: 30s
  scrapeTimeout: 30s

Checklist

• DCO signed
• Chart Version bumped
• Title of the PR starts with chart name (e.g. [prometheus-couchdb-exporter])