Aggregated breakdown of response metadata (utm_source, referrer, landing-page, and any custom keys attached to responses), computed server-side.

Call without key to discover which keys exist and how many responses carry each. Call with key to get that key's top values, and — when a slideId is also given — each value broken down by the answers those respondents gave. Counts are per selected option, so a question accepting several answers is not counted as one bucket per combination. truncated is true when the value list was cut at limit.