Simplify MetricManager

Investigate how to simplify MetricManager, see https://gitlab.teklia.com/atr/dan/-/blob/1956acc9bbe79f6ce1b8ffba69fd3fe16cb49571/dan/manager/metrics.py.

changed milestone to %DAN-P5: Improve training

added P2 label

mentioned in merge request !249 (merged)

assigned to @yschneider

closed with merge request !249 (merged)