Add cohort metric type for two-level aggregation (#124) #582

Job	Run time
snowflake-integration	21s
clickhouse-integration	44s
ADBC integration (clickhouse)	44s
postgres-integration	35s
ADBC integration (postgres)	50s
ADBC integration (snowflake)	47s
bigquery-integration	24s
ADBC integration (bigquery)	1m 3s
	5m 28s

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