[Lens] Runtime fields should show in fields list

[wylieconlon]

Because runtime fields are not returned in _source, but are returned in fields, the solution to this issue is likely the same as improving the field existence check in general.

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)

[AlonaNadler]

Ideally, runtime fields in Lens :

• Show up in the left side field list, without doing anything explicitly by the user
• Users can drag and drop a runtime field and to the preview or configurator and visualize based on the runtime fields values
• When selected a runtime field on the left side it shows a preview

[wylieconlon]

@AlonaNadler This issue is only about the first point in your list, based on investigation by @stacey-gammon. I would expect points 2 and 3 to work automatically. If there are bugs in points 2 and 3, let's open a separate issue to track the bugs.

[stacey-gammon]

How can a user drag and drop a runtime field if it doesn't show up on the left field list?

[wylieconlon]

I just tested a runtime field using the example from this PR, and found that the only issue I could identify is the one listed here. The runtime field shows up in the list of "empty fields" instead of "available fields", but still has data and is able to be dragged: