In order for code in OSS Kibana to create their own collector objects and use the collector set, these classes should be moved out of monitoring and into OSS Kibana. We don't want cross OSS/X-Pack imports.

Source of the classes: https://github.com/elastic/kibana/tree/744fc0b/x-pack/plugins/monitoring/server/kibana_monitoring/classes

We could even have the collectorSet that gets instantiated exposed somewhere else on the server object, instead of exposed as server.plugins.monitoring.collectorSet

cc @pickypg @nreese