So how do we measure it?
I have yet to see any meaningful increase in a team’s productivity after they start tracking “developer productivity”.
Each time it results in a blow to developer morale and a pretty dashboard that management uses to retroactively justify their decisions.
If you want some form of rational decision when you develop a team, how else but with metrics do you make decisions?
It's a terrible answer, in no small part because it's true.
How do you manage people when you're bad at managing people and can't/won't look at it? You manage safe, crisp, numbers instead.
Now imagine who you want to pay to do stuff and how to measure them. The best way is to see their actual performance unfold before your eyes and use your own intuition. See who gets things done and who doesn't. Who needs more handholding and who can work more independently. Some workers are much better "deals" than others from a company perspective.
You're completely failing to answer the question.
The best physics professor I ever had assigned a fuckload of homework every single day of class and graded it all himself. He probably worked 65+ hours a week, minimum. But you got feedback on all homework and that gave you an understanding of how well you were grasping concepts and on what you needed to work.
How can a manager do code review when he doesn't even have the capacity to know the details of the tasks?