JobStats does not handle nulls.

[johnynek]

Should probably be reworked a bit. Something like:

trait JobStats {
  def totals: Map[StatName, Long]
  def steps: Seq[JobStats]
}

And then we can define a monoid on them and just add them up. Also, I'm not sure exposing the steps is the right thing to do. Should you really be thinking in terms of steps? How do you reason about that? At the application layer, it seems to me you should use different keys to distinguish, not steps.