Benp/add total event count

@rocha @brianhw @mulby 

I think it makes sense to get started on review. For now, could you ignore (or at most, skim) the test files? 

The prod code is worth a closer look at this point, and I welcome any feedback.
Also, we can take advantage of a combiner here to save data over the wire: you just have to put `combiner = reducer` in the class definition and it will use the reducer as a combiner.