Lists the fields a metric event can filter, group and aggregate by, with what each supports.

Building an event from the response:

• Filter: a field with filterable: true becomes a filter
  {"column": field, "comparison": one of comparisons, "value": ...}. Copy field_type
  (and filter_type when set) onto the filter. in/not_in take a list. When values
  is not empty, only those values are accepted; when value_source is set, the field
  holds IDs listed by that endpoint.
• Group by: a field with groupable: true becomes {"field": field} in group_by.
• Aggregate: a field with aggregatable: true becomes
  {"aggregate_type": one of aggregate_types, "field": field, "field_type": field_type},
  plus "analysis_field": true when the field has analysis_field: true.
  metadata.event_aggregate_types are the aggregates that take no field.

For non_custom reports, fields with required: true must be filtered exactly once, and
max_values caps the values of an in filter.

For custom_object_record.* events, pass custom_data_type_id to include that custom
object type's properties.