That does leave one last category, which is people who work on improving developer productivity. If it's your job to help other developers move more quickly, how do you measure that?
Well, first off, most people who work on developer productivity do have some specific product. Either they work on a test framework (which you would measure in a similar fashion to how you would measure a library) or they work on some tool that developers use, in which case you would measure something about the success or usage of that tool.
For example, one thing the developers of a bug tracking system might want to measure is number of bugs successfully and rapidly resolved. Of course, you would modify that to take into account how the tool was being used in the company – maybe some entries in the bug tracker are intended to live for a long time, so you would measure those entries some other way. In general, you'd ask: what is the product or result that we...