Ask HN: How do you measure developers productivity?

Hey guys, i am wondering, how do you measure productivity of developers in your team? I have been using RescueTime app but is too general, not focused on developers. Also, i tried WakaTime plugin into code editor, but it is saying a much... just general stats. Any tips which data collect? Or how to measure collected data? Eg. number of commits a day, number of pull requests? Lines added vs lines removed? Etc?

Thanks in advance George