Returns a dashboard and runs it, returning the generated data — one result entry per metric.

Both built-in and saved dashboards are retrieved this way: built-in dashboards resolve by
their stable reserved IDs (prefix 00000000-0000-4000-, discoverable via List Dashboards)
and return null created_at/updated_at. Each of their result entries carries the
metric_id of the corresponding built-in metric.

Optionally provide an absolute date range to generate the dashboard over:

• start - range start in ISO 8601 format.
• end - range end in ISO 8601 format.
• timezone - IANA timezone the range is interpreted in (defaults to America/Chicago).

start and end must be supplied together. When omitted, a saved dashboard uses its stored
timeframe; a built-in dashboard runs over the trailing four weeks.

For built-in time-series metrics, ranges longer than six months are bucketed by week instead
of hour/day to keep result sizes bounded.