feat(postgres): Add alert for WAL size

infrastructure/postgres/kustomization.yaml

@@ -6,6 +6,7 @@ resources:
   - repository.yaml
   - release.yaml
   - podmonitor.yaml
+  - prometheusrules.yaml
 configMapGenerator:
   - name: postgres-system-grafana-dashboards
     files:

infrastructure/postgres/prometheusrules.yaml

+---
+apiVersion: monitoring.coreos.com/v1
+kind: PrometheusRule
+metadata:
+  name: postgres-rules
+  namespace: postgres-system
+spec:
+  groups:
+  - name: postgres-clusters
+    rules:
+    - alert: PostgresHighWALUsage
+      annotations:
+        description: The WAL size of the postgres cluster exceeded 1GiB for more than 1 hour. This indicates a problem with either a replica or the WAL archiving process to S3.
+        summary: The WAL size of the postgres cluster exceeded 1GiB for more than 1 hour.
+      expr: max by (namespace, cluster_name)(pg_wal_size_bytes) > 2^30
+      for: 1h
+      labels:
+        issue: The WAL size of the postgres cluster exceeded 1GiB for more than 1 hour.
+        severity: critical
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