Is it possible to fire a trigger per key?

[Andre Marianiello]

 Based on my current understanding, when a trigger fires, all elements that
have been waiting behind a GroupByKey are allowed to continue through the
pipeline. Is it possible to create a custom trigger that fires
independently for each key? For example, instead of a trigger that fires
after 1000 elements total, is it possible to write a trigger that fires for
a specific key once 1000 elements with that key have been found?