fix(monitoring): Allow exposure through ingress

This patch allows network requests from the ingress namespace(s) by
adding our shared network policy.

infrastructure/monitoring/kustomization.yaml

@@ -6,3 +6,6 @@ resources:
   - repository.yaml
   - release.yaml
   - ../../shared/networkpolicies/allow-from-same-namespace.yaml
+  - ../../shared/networkpolicies/allow-from-ingress.yaml
+patchesStrategicMerge:
+  - networkpolicy.yaml

infrastructure/monitoring/networkpolicy.yaml

+---
+apiVersion: networking.k8s.io/v1
+kind: NetworkPolicy
+metadata:
+  name: allow-from-ingress
+spec:
+  podSelector:
+    matchExpressions:
+      - key: app.kubernetes.io/name
+        operator: In
+        values:
+          - alertmanager
+          - grafana
+          - prometheus