I have always been a little suspicious of metrics as a way to measure people. We have all seen how people can "game" a measurement. For example, measuring software developers by lines of code written can cause developers to write more "verbose" code. When this happens the measurement stops working since people adjust to meet the goal. Worse though, is when the gaming creates unintended consequences. In our example, senior software developers can stop coaching the junior developers to have time to focus on their own lines of code. That is certainly not a positive result.
An article today on BoingBoing describes Goodhart's Law, which essentially says that once you start controlling something with a measure, the value of the measure diminishes. Some measurement is unavoidable, but where you do measure watch for Goodhart's law and other unintended consequences.