monitoring: Elaborate more about operator

monitoring/README.md

@@ -1,11 +1,26 @@
-## Monitoring
+## Monitoring namespace
 
 Prometheus is accessible at [prom.k-space.ee](https://prom.k-space.ee/)
 and the corresponding AlertManager is accessible at [am.k-space.ee](https://am.k-space.ee/).
 Both are [deployed by ArgoCD](https://argocd.k-space.ee/applications/monitoring)
 from this Git repo directory using Prometheus operator.
 
-Alerts are sent to #kube-prod Slack channel
+Note that Prometheus and other monitoring stack components should use the
+`dedicated: monitoring` Kubernetes node selector to make sure the components
+get scheduled on `mon[1-3]` nodes which are hosted in a privileged VLAN where
+they have access to UPS SNMP targets, Mikrotik router/switch API-s etc.
+
+To add monitoring targets inside the Kubernetes cluster make use of
+[PodMonitor](https://github.com/prometheus-operator/prometheus-operator/blob/main/Documentation/user-guides/getting-started.md#using-podmonitors) or ServiceMonitor custom
+resource definitions.
+
+For external targets (ab)use the Probe CRD as seen in `node-exporter.yaml`
+or `ping-exporter.yaml`
+
+Alerts are sent to #kube-prod Slack channel. The alerting rules are automatically
+picked up by Prometheus operator via Kubernetes manifests utilizing
+the operator's
+[PrometheusRule](https://github.com/prometheus-operator/prometheus-operator/blob/main/Documentation/user-guides/alerting.md#deploying-prometheus-rules) custom resource definitions.
 
 Sample queries: