Update kube-prometheus-on-kubeadm.md

In kubeadm 1.10 (and perhaps earlier versions, not sure), ctrl-mgr and scheduler pods' label key is `component=` rather than `k8s-app=`, which is the selector key used for kube-prometheus exporters for these components.

docs/kube-prometheus-on-kubeadm.md

@@ -42,7 +42,7 @@ schedulerExtraArgs:
   address: 0.0.0.0
 ```
 
-Notice the `schedulerExtraArgs` and `controllerManagerExtraArgs`. This exposes the `kube-controller-manager` and `kube-scheduler` services to the rest of the cluster.
+Notice the `schedulerExtraArgs` and `controllerManagerExtraArgs`. This exposes the `kube-controller-manager` and `kube-scheduler` services to the rest of the cluster. If you have kubernetes core components as pods in the kube-system namespace, ensure that the `kube-prometheus-exporter-kube-scheduler` and `kube-prometheus-exporter-kube-controller-manager` services' `spec.selector` values match those of pods.
 
 In addition, we will be using `node-exporter` to monitor the `cAdvisor` service on all the nodes. This, however requires a change to the `kubelet` service on the master as well as all the nodes. According to the Kubernetes documentation