Metric aggregation (#3321)

* metric aggregation

* metric aggregation

* add at_least_1d

* fix output formatting

* add metric tests

* add missing test case

* remove reduce_op frm metric classes

* fix reduce_op stuff

* start test fixing

* fix tests due to aggregation

* fix faulty import

* Apply suggestions from code review

Co-authored-by: Adrian Wälchli <aedu.waelchli@gmail.com>

* remove reduce_op docstrings

* add compute

* remove import

* remove collection metric

* update base class

* update tests

* Update metric.py

* Update metric.py

* Apply suggestions from code review

* change default aggregate

Co-authored-by: Adrian Wälchli <aedu.waelchli@gmail.com>
Co-authored-by: Jirka Borovec <Borda@users.noreply.github.com>
Co-authored-by: Nicki Skafte <skaftenicki@gmail.com>