Productivity to me is a measure of cost vs impact, where cost for developers is almost always time. Time spent is easy to measure - something like time from idea to deliverable is usually good enough. I don't think you need to be precise. You just need a general idea of what kind of work takes ages, and what can be done quickly.
Measuring impact is trickier. It should be a signal of how well you're achieving the goal of your project. If you're working on, say, a budgeting tool, you will want to measure how much money your customers have managed to save, or something along those lines.
