@spec get_revenue_tracking_currency(Plausible.Site.t(), Plausible.Stats.Query.t(), [
  atom()
]) ::
  {atom() | nil, [atom()]}

Returns the common currency for the goal filters in a query. If there are no goal filters, multiple currencies or the site owner does not have access to revenue goals, nil is returned and revenue metrics are dropped.

Aggregating revenue data works only for same currency goals. If the query is filtered by goals with different currencies, for example, one USD and other EUR, revenue metrics are dropped.