FILTER clause for aggregate functions is ignored

Drill ignores FILTER clause for aggregate functions.
The following query returns the wrong result:

select count(*), count(employee_id) filter(where employee_id < 5) from cp.`employee.json`;

+--------+--------+
| EXPR$0 | EXPR$1 |
+--------+--------+
| 1155 | 1155 |
+--------+--------+

Calcite already supports this feature (CALCITE-704) and as it is mentioned in the Jira ticket, such syntax is allowed by SQL standard.

As a short solution, we should throw an exception for such queries that this functionality is not supported.
As was mentioned in Calcite's Jira, some queries may be rewritten using switch case:

select count(*), count(case when employee_id < 5 then employee_id else null end) from cp.`employee.json`;

It is possible to add functionality into Drill to rewrite filtered aggregate calls in such a way, but some aggregate functions still would not be supported, for example, count(*).