Currently supported by PostgreSQL 9.4+ and SQLite 3.30+

See:

 - http://modern-sql.com/feature/filter
 - https://www.postgresql.org/docs/9.4/static/sql-expressions.html#SYNTAX-AGGREGATES
 - https://sqlite.org/lang_aggfunc.html#aggfilter

Example:

    Model.all.pluck(
      Arel.star.count.as('records_total').to_sql,
      Arel.star.count.filter(Model.arel_table[:some_column].not_eq(nil)).as('records_filtered').to_sql,
    )