[Lens] Render counter and gauge metrics as separate field types in Lens field list

[flash1293]

The filter menu in Lens should be reduced in size according to the screenshot and gauge and counter metrics should be split out as separate field types in the type selector

[elasticmachine]

Pinging @elastic/kibana-vis-editors @elastic/kibana-vis-editors-external (Team:VisEditors)

[stratoula]

@flash1293 this also applies for discover field list?

[flash1293]

Good question @stratoula. I don't think it matters much in the first release as you can't really use these fields effectively in Discover anyway. Once we start consolidating the field lists they should behave identical, but I think we can live with this difference for a minor or two - not worth doing the changes twice right now.