Hi there, I'm struggling to setup an active days metric via a fact table. I am extracting data from a Snowplow sessions table. I have added a day column, but I'm not able to count it in the fact table metrics editor. Can someone help me out please?

SELECT
  user_id,
  domain_userid as anonymous_id,
  start_tstamp as timestamp,
  date_trunc(start_tstamp, Day) as day
FROM
  `xxx`.`marts`.sessions_extended
where
    app_id in ("xxx")
    and
    date(start_tstamp) BETWEEN
    '{{date startDateISO "yyyy-MM-dd"}}' AND '{{date endDateISO "yyyy-MM-dd"}}'