doc/user-guide.md: Link to metrics docs

Author: lilic

doc/user-guide.md

@@ -400,6 +400,10 @@ func main() {
 
 After adding new import paths to your operator project, run `dep ensure` in the root of your project directory to fulfill these dependencies.
 
+### Metrics
+
+To learn about how metrics works in operator-sdk read the [metrics section][metrics_doc] of the user documentation.
+
 [memcached_handler]: ../example/memcached-operator/handler.go.tmpl
 [memcached_controller]: ../example/memcached-operator/memcached_controller.go.tmpl
 [layout_doc]:./project_layout.md
@@ -419,3 +423,4 @@ After adding new import paths to your operator project, run `dep ensure` in the
 [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