It is sort of measurable. How many lines you output, how many bugs are in your code, how much time is spent fixing your bugs. Etc. I mean its far from perfect and leadership roles are harder to measure (but can be measured by how the people under them think about them). But still I get your point.

I think the best we can do is have an expert familiar with the task and codebase evaluate contributions. Trouble is, this can be corrupted by all sorts of internal politicking.