Lists the events a metric can be built from, with the charts each supports. Use it with
List Metric Event Fields to build a metric's events.

• whippy_event — events Whippy records for the organization (e.g. call.analyzed).
• custom_event — events the organization sends to Whippy.
• non_custom — reports defined by Whippy (e.g. new_conversations). These take only the
  filters and group-by fields List Metric Event Fields returns for them.

Whippy and custom events are the ones the organization recorded in the last year
(seen_recently: true). Pass include_inactive=true to also list Whippy events the
organization hasn't recorded yet, to set a metric up before the data arrives.