Get a list of values for a metadata field

Implement an API method(s) to get a list of values for a metadata field. For a field with discrete values, this API method might just return all known values. However, for a field with continuous values, the API method should return a set of ranges of values (bins) where the number of records that fall within each range are roughly equivalent (similar to Pandas [qcut](https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.qcut.html) function). The API method should provide a parameter for how many bins to return for continuous values.

We might want to add a "continuous or discrete" attribute to metadata fields API since this seems like an important thing to describe about a metadata field.

Also, when it comes to continuous data, that only applies to continuous data types like numeric and date-time data types.

Start by locating the metadata fields API and reviewing how field types are currently represented. Define the API behavior for discrete fields, continuous numeric or date-time fields, and the requested bin count. Done means the contract clearly supports listing known values or returning approximately equal-frequency ranges.

Written by the indexing model from the issue text.