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, the field takes one of those values; 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 call.analyzed analysis fields (analysis_field: true), values is what agents are
configured to produce: the combined enums of every agent in defined_by_agents.
observed_values is what the organization's calls actually recorded; build filters from it
when it is not empty, since configured values a call never recorded match nothing.
defined_by_agents lists the agents whose current version defines the field, oldest first,
including archived and deleted ones. When agents disagree on a field's type, the oldest
agent's type is used. Fields with system_field: true are scored on every call rather than
defined by an agent, so their defined_by_agents is empty.

observed_values is refreshed daily and has known gaps: it keeps the newest 100 values
and never drops any, so values agents no longer produce stay listed; and it is empty for
system fields, booleans and numbers, which aren't collected.

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

For form_response.* events, pass form_id (as listed by List Forms) to include that form's
rating and scale questions as answer:<element_id> fields. Duplicated forms share element
ids, so an event that filters, groups or aggregates by an answer field must also filter
form_id to that single form (eq, or in with one value).