add interval to prometheus service monitor

We have a use-case where we need to modify the default scrape interval
on a number of apps, including coredns.

This PR simply adds support to change it. (It should be backwards
compatible with existing implmentations.)

Test/verification:
```
$ helm template charts/coredns --set prometheus.monitor.enabled=true --set prometheus.monitor.interval=60s |grep servicemonitor.yaml -A 22
apiVersion: monitoring.coreos.com/v1
kind: ServiceMonitor
metadata:
  name: RELEASE-NAME-coredns
  labels:
    app.kubernetes.io/managed-by: "Helm"
    app.kubernetes.io/instance: "RELEASE-NAME"
    helm.sh/chart: "coredns-1.16.5"
    k8s-app: "coredns"
    kubernetes.io/cluster-service: "true"
    kubernetes.io/name: "CoreDNS"
    app.kubernetes.io/name: coredns
spec:
  selector:
    matchLabels:
      app.kubernetes.io/instance: "RELEASE-NAME"
      k8s-app: "coredns"
      app.kubernetes.io/name: coredns
      app.kubernetes.io/component: metrics
  endpoints:
    - port: metrics
      interval: 60s
```

Signed-off-by: Craig Sebenik <csebenik@aurora.tech>

charts/coredns/README.md

@@ -63,6 +63,7 @@ The command removes all the Kubernetes components associated with the chart and
 | `prometheus.monitor.enabled`                    | Set this to `true` to create ServiceMonitor for Prometheus operator                                                                       | `false`                                                     |
 | `prometheus.monitor.additionalLabels`           | Additional labels that can be used so ServiceMonitor will be discovered by Prometheus                                                     | {}                                                          |
 | `prometheus.monitor.namespace`                  | Selector to select which namespaces the Endpoints objects are discovered from.                                                            | `""`                                                        |
+| `prometheus.monitor.interval`                   | Scrape interval for polling the metrics endpoint. (E.g. "30s")                                                                            | `""`                                                        |
 | `service.clusterIP`                             | IP address to assign to service                                                                                                           | `""`                                                        |
 | `service.loadBalancerIP`                        | IP address to assign to load balancer (if supported)                                                                                      | `""`                                                        |
 | `service.externalIPs`                           | External IP addresses                                                                                                                     | []                                                          |

charts/coredns/templates/servicemonitor.yaml

@@ -30,4 +30,7 @@ spec:
       app.kubernetes.io/component: metrics
   endpoints:
     - port: metrics
+      {{- if .Values.prometheus.monitor.interval }}
+      interval: {{ .Values.prometheus.monitor.interval }}
+      {{- end }}
 {{- end }}

charts/coredns/values.yaml

@@ -63,6 +63,7 @@ prometheus:
     enabled: false
     additionalLabels: {}
     namespace: ""
+    interval: ""
 
 service:
 # clusterIP: ""