add option for custom aggregation

[lintangsutawika]

No description provided.

[LSinev]

As a suggestion — also edit metric name in reports and logs.
In "the old days" of python tasks one could make, for example, F1 aggregated 'macro' and name metric F1_macro which was logged and reported. Now, with yaml tasks, one can make F1 with macro (or any other, more complex) but it will still be named f1. With this custom aggregation, custom naming may also be considered.

[lintangsutawika]

That's a great idea. I will make adjustments