Updates a metric saved in the organization. Only the fields sent change; events is
replaced as a whole when sent. A metric read with Show Metric or List Metrics can be sent
back as is.

Built-in metrics (source: whippy) can't be updated.

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.