doc/user-guide.md: Link metrics

Author: lilic

doc/user-guide.md

@@ -460,6 +460,10 @@ func main() {
 
 When the operator is not running in a cluster, the Manager will return an error on starting since it can't detect the operator's namespace in order to create the configmap for leader election. You can override this namespace by setting the Manager's `LeaderElectionNamespace` option.
 
+### Metrics
+
+To learn about how metrics works in operator-sdk read the [metrics section][metrics_doc] of the user documentation.
+
 
 [pod_eviction_timeout]: https://kubernetes.io/docs/reference/command-line-tools-reference/kube-controller-manager/#options
 [manager_options]: https://godoc.org/github.com/kubernetes-sigs/controller-runtime/pkg/manager#Options
@@ -485,3 +489,4 @@ When the operator is not running in a cluster, the Manager will return an error
 [controller-go-doc]: https://godoc.org/github.com/kubernetes-sigs/controller-runtime/pkg#hdr-Controller
 [request-go-doc]: https://godoc.org/github.com/kubernetes-sigs/controller-runtime/pkg/reconcile#Request
 [result_go_doc]: https://godoc.org/github.com/kubernetes-sigs/controller-runtime/pkg/reconcile#Result
+[metrics_doc]: ./user/metrics/README.md