Enable Prometheus scraping (#380)

## Description

Fixes #379

CHANGELOG.md

@@ -6,6 +6,7 @@ All notable changes to this project will be documented in this file.
 
 ### Added
 
+- Enabled Prometheus scraping ([#380]).
 - ZookeeperZnode.spec.clusterRef.namespace now defaults to .metadata.namespace ([#382]).
 
 ### Changed
@@ -17,6 +18,7 @@ All notable changes to this project will be documented in this file.
 [#338]: https://github.com/stackabletech/zookeeper-operator/pull/338
 [#340]: https://github.com/stackabletech/zookeeper-operator/pull/340
 [#352]: https://github.com/stackabletech/zookeeper-operator/pull/352
+[#380]: https://github.com/stackabletech/zookeeper-operator/pull/380
 [#382]: https://github.com/stackabletech/zookeeper-operator/pull/382
 
 ## [0.8.0] - 2021-12-22

docs/modules/ROOT/pages/usage.adoc

@@ -41,3 +41,7 @@ Finally, a ConfigMap is created, containing a path that a ZooKeeper client can c
     $ kubectl get configmap simple-znode-nodeport -o yaml
     $ $ZOOKEEPER_HOME/bin/zkCli.sh -server $(kubectl get configmap simple-znode-nodeport -o jsonpath='{.data.ZOOKEEPER}')
 
+== Monitoring
+
+The managed ZooKeeper instances are automatically configured to export Prometheus metrics. See
+xref:home::monitoring.adoc[] for more details.

rust/operator-binary/src/zk_controller.rs

@@ -317,6 +317,7 @@ fn build_server_rolegroup_service(
                 &rolegroup.role,
                 &rolegroup.role_group,
             )
+            .with_label("prometheus.io/scrape", "true")
             .build(),
         spec: Some(ServiceSpec {
             cluster_ip: Some("None".to_string()),