Add monitoring example

docs/examples/prometheus/README.md

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+# Prometheus Example
+
+See [Monitoring RabbitMQ in Kubernetes](https://www.rabbitmq.com/kubernetes/operator/operator-monitoring.html) for detailed instructions.
+
+If you deployed the [Prometheus Operator](https://github.com/prometheus-operator/prometheus-operator), make Prometheus scrape RabbitMQ nodes by:
+```shell
+kubectl apply -f rabbitmq-podmonitor.yaml
+```
+
+Alternatively, if you deployed the Prometheus Operator via the [kube-prometheus-stack](https://github.com/prometheus-community/helm-charts/tree/main/charts/kube-prometheus-stack) Helm chart,
+set the values in [kube-prometheus-stack-values.yaml](kube-prometheus-stack-values.yaml) when installing / upgrading the Helm chart.

docs/examples/prometheus/kube-prometheus-stack-values.yaml

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+---
+prometheus:
+  additionalPodMonitors:
+   - name: rabbitmq
+     podMetricsEndpoints:
+     - port: prometheus
+     selector:
+       matchLabels:
+         app.kubernetes.io/component: rabbitmq
+     namespaceSelector:
+       any: true

docs/examples/prometheus/rabbitmq-podmonitor.yaml

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+---
+apiVersion: monitoring.coreos.com/v1
+kind: PodMonitor
+metadata:
+  name: rabbitmq
+spec:
+  podMetricsEndpoints:
+  - interval: 15s
+    port: prometheus
+  selector:
+    matchLabels:
+      app.kubernetes.io/component: rabbitmq
+  namespaceSelector:
+    any: true