Add prometheus servicemonitor support for NGINX Ingress controller

[jasonwilliams14]

This enhancement is to support prometheus servicemonitor for NGINX Ingress controller helm deployments.

prometheus-operator Github

References:

#889
#3129

Being able to leverage the servicemonitor CRD that prometheus provides, allows users to have more flexibility and control when collecting metrics for NGINX Ingress controller.

This request will add support for both helm and manifest deployments, allowing customer to enable (helm) or apply (manifest) the additional services and servicemonitor that are required.

When using helm, if the user opts to enable servicemonitor the deployment will generate the appropriate service and servicemonitor required.
Example manifests (thanks @hostalp)

Additional service required:

apiVersion: v1
kind: Service
metadata:
  name: nginx-ingress-nginx-ingress-servicemonitor
  labels:
    app: nginx-ingress-nginx-ingress-servicemonitor
spec:
  ports:
  - name: prometheus
    protocol: TCP
    port: 9113
    targetPort: 9113
  selector:
    app: nginx-ingress-nginx-ingress

Servicemonitor definition:

apiVersion: monitoring.coreos.com/v1
kind: ServiceMonitor
metadata:
  name: nginx-ingress-nginx-ingress-servicemonitor
  labels:
    app: nginx-ingress-nginx-ingress-servicemonitor
    release: kube-prometheus-stack
spec:
  selector:
    matchLabels:
      app: nginx-ingress-nginx-ingress-servicemonitor
  endpoints:
  - port: prometheus

The user will have to have ensure they have prometheus installed and running correctly in there cluster in order for servicemonitor to work correctly.

### Tasks
- [ ] https://github.com/nginxinc/kubernetes-ingress/issues/4329
- [ ] https://github.com/nginxinc/kubernetes-ingress/issues/4328
- [ ] https://github.com/nginxinc/kubernetes-ingress/issues/4330

[AaronShiels]

Looks like exactly what is needed to finally get out-of-the-box Prometheus metrics!

[jasonwilliams14]

Attached drawing to show connections for service, servicemonitor and prometheus.