Runs a metric definition without saving it and returns its generated data. A definition
that previews successfully can be created as is: preview accepts exactly what Create
Metric does (title is optional), and returns the same results Show Metric returns for
the saved metric over the same range.

The window is set like Show Metric's: start and end (ISO 8601, together) in
timezone (default America/Chicago), or the trailing four weeks when omitted.

A metric is a chart (chart) over one or more events (events). Build events with List
Metric Events and List Metric Event Fields:

• type and name come from List Metric Events, which also lists the charts each supports.
• filter, group_by and aggregate use the fields List Metric Event Fields returns.
• non_custom events (and a metric type of non_custom) name a report; only the reports
  List Metric Events lists are accepted, with the filters and group-by fields it describes.

Rules that span events: linked metrics (linked: true) have exactly two events, one
whippy_event and one custom_event, with different ranks; funnel steps are ordered by
rank; heatmaps take no aggregate; bar, column and line charts group by at most one field;
group_colors and event color take #RRGGBB hex colors.