Detect and warn about possible duplication targets

[petewall]

The most common source of metric duplication is because of the combination of:

1. The Cluster Metrics feature
2. ServiceMonitors already existing that scrape an existing kube-state-metrics, kubelet, cadvisor, node exporter.

We should try to detect a few of these cases and warn in the NOTES.tpl (not an error, but a warning).