I actually think that defining the metrics is half of the game for productivity. Once you define the metrics, it's relatively easy to game it.

That said, I think software development is a relatively well defined task. There's typically some clear need from clients, and there's some restriction. Your problem space is somewhat confined.

Now, corollary is this: the more restriction you have, the easier you can be productive (or invent a way to be productive). Specialists can be generally more productive than generalists because they can have a more precise definition of "being productive".