Need public constructor and settable properties on RegressionMetrics and their sibling classes

AutoML API returns Metric for each hypothesis it builds and runs.
When data is small autoML API internally does cross validate (instead of train validate) and the metric it wants to return is the average of the metric from training each fold.
Currently it is not possible to build a new metric object and populate it with average values since the RegressionMetric and their siblings etc don't have public constructor.

Can we please provide a public constructor to these Metric classes?
It will be great if we can accomadate for 0.12 release.

@TomFinley @shauheen