Going through the same process myself, short answer there is no easy metric that you can add that really quantifies a developer.

I would start measuring projects or teams first, getting a solid set of metrics for each and once you have that you can start tracking down individual developers.

Even so tracking developer productivity can be hard, a good chunk of our work happens in our head. Who is more valuable the developer that wrote thousands of lines of code or the one that changed one but fixed a critical bug in production?

I highly recommended this book http://shop.oreilly.com/product/0636920020134.do