Hence this being a proxy measurement. But managers really want a way to measure "developer productivity", and the feeling is that proxy measurements are better

Hence this being a proxy measurement. But managers really want a way to measure "developer productivity", and the feeling is that proxy measurements are better than nothing, even though they aren't. (Two simple examples: (1) how do you measure the productivity of a lead developer who spends much of his time mentoring, pairing, and helping the team deliver working code? (2) Is a developer who commits many small PR's "worth more" to the company than a developer who commits fewer, larger PR's?)