Break down a metric by a dimension.

having[]=metric:op:value (op: gt | gte | lt | lte | eq | neq; value a plain decimal; at most 2 conditions, ANDed) keeps only the groups whose having-metric — at its default measurement, over the same grouped rows — passes; it applies before ordering and limit. A group whose having-metric is NULL is dropped by every op. Session metrics (feed_sessions, avg_session_depth, avg_session_duration) are rejected as having metrics (400).