[Discover] Add Sample Size Selection to Field Statistics

[jeffvestal]

Describe the feature:
Add a drop down option in the Field Statistics view to allow increasing the sample size.
Currently this exists in the Data Visualizer

Describe a specific use case for the feature:
Setting a smaller sample size is often good enough balance of speed and information, but there are times when it is important to be able to sample a larger set of documents, or all documents, to get a more accurate distribution view.

For example, I need to know the true cardinality of a field, the initial sample size showed 2 but the true carnality was 9. In the Data Visualizer I can select "Search all" and get the accurate answer.

[elasticmachine]

Pinging @elastic/kibana-data-discovery (Team:DataDiscovery)

[elasticmachine]

Pinging @elastic/ml-ui (:ml)

[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.