Add ServiceMonitor support

[sergeyshaykhullin]

monitoring/prometheus/...:
  serviceMonitor:
    enabled: ...
    ...etc

There are some examples:

• https://hub.helm.sh/charts/argo/argo-cd
• https://hub.helm.sh/charts/stable/grafana
• https://hub.helm.sh/charts/jaegertracing/jaeger-operator
• https://hub.helm.sh/charts/bitnami/prometheus-operator
• https://hub.helm.sh/charts/stable/rabbitmq-ha
• https://hub.helm.sh/charts/jetstack/cert-manager
• https://hub.helm.sh/charts/stable/nginx-ingress
• https://hub.helm.sh/charts/loki/loki
• https://hub.helm.sh/charts/loki/promtail
• https://hub.helm.sh/charts/stable/redis-ha