fix(customPageEditor): MON-77 reset metric label, unit and aggregation on variable change

Picking another variable kept the previous title, unit, holding aggregation and format, so a card switched from fat to total milk still showed "%" and averaged the milk over the holding. None of these fields can be fixed in the UI, and the backend takes them from the reference as is. The replacement is built explicitly so future MetricRef fields do not silently move to another variable; only the donut slot colour stays.