Druid support for theta sketch

- My data is in a Druid database.
- My Metabase version is v0.19.1.

It looks like theta sketch metrics are not supported: http://druid.io/docs/latest/development/extensions-core/datasketches-aggregators.html

Theta sketches is a very useful Druid metric type, as it supports many set operations that enable cardinality counts on large data sets.

In Metabase the type for this theta sketch columns is `FloatField`, it should be something like `ThetaSketch`. It also looks like it isn't counted as a metric, just as a dimension.

Start by tracing Metabase's handling of Druid field types and its distinction between metrics and dimensions, using the linked Druid theta-sketch aggregator documentation for expected behavior. Done means theta sketch columns are represented distinctly from FloatField and are available as metrics with the relevant set operations.

Written by the indexing model from the issue text.