Add Prometheus config for monitoring Kubernetes Ingress

* Allow Kubernetes Ingress resources to be probed via Blackbox
Exporter (if present) if annotated `prometheus.io/probe: "true"`
* Fix probes of Services via Blackbox Exporter. Require Blackbox
Exporter to be deployed in the same `monitoring` namespace, be
named `blackbox-exporter`, and use port 8080

addons/prometheus/rbac/cluster-role.yaml

@@ -10,6 +10,17 @@ rules:
   - services
   - endpoints
   - pods
-  verbs: ["get", "list", "watch"]
+  verbs:
+  - get
+  - list
+  - watch
 - nonResourceURLs: ["/metrics"]
   verbs: ["get"]
+- apiGroups:
+  - networking.k8s.io
+  resources:
+  - ingresses
+  verbs:
+  - get
+  - list
+  - watch