After having read #20, it might be possible to auto-register appengine-counter entities with Objectify.
It seems like there's only ever a single ObjectifyFactory floating around which is typically access in a static fashion, so it might make sense to add another constructor to ShardedCounterServiceImpl that requires an ObjectifyFactory. That way, when you construct the ShardedCounterService (or when your DI system does) appengine-counter could auto-register its entities.
This needs to be experimented with first to see if there are any edge cases not being considered.