[Feature Request] Be able to group by database/retention policy/measurement

Hi there.

Is there a recommended way to run a cross database task ? Let's say I have two databases with the same measurement `page_count_status`.

I would like to use kapacitor to compute the success / rate (ie `page_count_status success / (page_count_status success + page_count_status failed`) for each database.

Do I need to create two tasks for this or is it a way to have one tasks for all DB ?

Thanks.

No files, tests, or entry points are named. Start by determining how Kapacitor tasks select databases, retention policies, and measurements, then define whether one task can calculate the requested success rate separately for each database; done means the behavior and supported configuration are clear.

Written by the indexing model from the issue text.