MetricTask is a Gen 2 task that provides a non-dataref abstract run method, as well as some functionality needed to read Gen 3 input specifications without access to a Gen 3 repository. As such, it should be possible to add PipelineTask functionality using the standard procedure developed for CmdLineTask.
Story points are a guess; I'm not clear what needs to be done besides adding a Connections class for the Measurement output. MetricTask should use the default runQuantum method, although at least one subclass will need to override it (
The existing Gen 2 methods like MetricTask.areInputDatasetsScalar should not be deprecated until Gen 2 itself is.