FEAT(alertmanager): add ServiceMonitor

- Create servicemonitor.yaml for Prometheus to scrape Alertmanager
- alertmanager chart does not include ServiceMonitor, must be added separately
- Enables Grafana Alertmanager dashboard to display data

alertmanager/kustomization.yaml

@@ -2,4 +2,5 @@ apiVersion: kustomize.config.k8s.io/v1beta1
 kind: Kustomization
 resources:
   - ingress.yaml
+  - servicemonitor.yaml
   - vault/alertmanager-secrets.yaml