[Lens] Field list - use variable_width_histogram in field stats endpoint

[wylieconlon]

Currently the field stats endpoint, which is used when clicking on a field on the left side of Lens, makes two sequential requests for number fields. The second request depends on the min/max numeric values, and the histogram bucketing is determined based on this. Instead, we could use the new variable-width histogram to do this in a single request.

There are two changes needed here:

1. Change the server side of Lens to make 1 request instead of 2
2. Change the chart rendering in the field stats popover to handle variable-width bars, most likely by using an area chart with a stepped fitting function instead of a bar chart. Bar charts can't handle variable-width bars.

[elasticmachine]

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

[stratoula]

Thank you for contributing to this issue, however, we are closing this issue due to inactivity as part of a backlog grooming effort. If you believe this feature/bug should still be considered, please reopen with a comment.