Returns a metric and runs it, returning the generated data.

Both built-in and saved metrics are retrieved this way: built-in metrics resolve by their
stable reserved IDs (prefix 00000000-0000-4000-, discoverable via List Metrics) and return
null created_at/updated_at.

Metrics have no stored timeframe, so the window is determined by an optional absolute date range:

• 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, the metric 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.