Allow to scrape etcd metrics using a service (#8203)

Signed-off-by: Mathieu Parent <math.parent@gmail.com>

docs/etcd.md

@@ -0,0 +1,30 @@
+# etcd
+
+## Metrics
+
+To expose metrics on a separate HTTP port, define it in the inventory with:
+
+```yaml
+etcd_metrics_port: 2381
+```
+
+To create a service `etcd-metrics` and associated endpoints in the `kube-system` namespace,
+define it's labels in the inventory with:
+
+```yaml
+etcd_metrics_service_labels:
+  k8s-app: etcd
+  app.kubernetes.io/managed-by: Kubespray
+  app: kube-prometheus-stack-kube-etcd
+  release: prometheus-stack
+```
+
+The last two labels in the above example allows to scrape the metrics from the
+[kube-prometheus-stack](https://github.com/prometheus-community/helm-charts/tree/main/charts/kube-prometheus-stack)
+chart with the following Helm `values.yaml` :
+
+```yaml
+kubeEtcd:
+  service:
+    enabled: false
+```