[Discover] Allow users to decide whether multi-fields appear in CSV export

[wylieconlon]

Discover now supports multi-fields like field, field.keyword when using the new fields API. When exporting the entire document to CSV, Discover now includes these multi-fields as separate columns. This will sometimes contain duplicate information, although it is not always duplicate. For example, if a field is multi-mapped as both text and keyword, the keyword field may have a character limit that truncates the data that's stored in Elasticsearch, while keeping the original text intact.

I propose that we add a checkbox to the Discover CSV export settings which will "Include fields that are multi-mapped". The default setting of this checkbox should be enabled.

cc @nreese for raising this as a topic

[elasticmachine]

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

[elasticmachine]

Pinging @elastic/kibana-reporting-services (Team:Reporting Services)

[kertal]

Closing this because it's not planned to be resolved in the foreseeable future. It will be tracked in our Icebox and will be re-opened if our priorities change. Feel free to re-open if you think it should be melted sooner.