Write a query to report the top-level payload keys present in purchase events in app_events. Return each key, the number of purchase events containing it, and pct as that count divided by all purchase events, multiplied by 100 and rounded to one decimal place.
Include only keys present in at least one purchase payload. A present key counts even when its value is JSON null. Ignore keys inside nested objects.
Ordering: Sort by events descending, then by key ascending.