Introduce metrics & servicemonitors

src/SUMMARY.md

@@ -9,3 +9,4 @@
 - [Containers](./containers/index.md)
   - [Kubernetes](./containers/k8s/index.md)
     - [RBAC](./containers/k8s/rbac.md)
+    - [Metrics & Service Monitors](./containers/k8s/metrics.md)

src/containers/k8s/metrics.md

@@ -0,0 +1,39 @@
+# Metrics & Service Monitors
+
+To keep track of the health of your services you can setup a [monitoring.coreos.com/v1.ServiceMonitor](https://prometheus-operator.dev/docs/api-reference/api/#monitoring.coreos.com/v1.ServiceMonitor) resource.
+
+This resource defines a service that will be monitored, and what endpoints, authentication, interval, etc to use.
+
+## Creating your first ServiceMonitor
+
+To create a ServiceMonitor you need to create a `ServiceMonitor` resource.
+
+```yaml
+apiVersion: monitoring.coreos.com/v1
+kind: ServiceMonitor
+metadata:
+  name: my-service-monitor
+  namespace: my-app
+spec:
+  selector:
+    matchLabels:
+      app: my-app
+  endpoints:
+    - port: http
+      interval: 30s
+      path: /metrics
+```
+
+The above service monitor will monitor the `http` port on the `my-app` namespace, and will scrape the `/metrics` endpoint every 30 seconds.
+
+## Service Implementation
+
+On the side of your service you will need to expose an http endpoint (generally `/metrics`) that returns the metrics in a format that Prometheus can scrape.
+
+An example of what this output might look like is:
+
+```yaml
+# HELP my_metric_1 A metric
+# TYPE my_metric_1 gauge
+my_metric_1 1.0
+```