[mikekchar]

That example is a measurement, not a metric (even though it is mistakenly called a metric by people who don't study this stuff). It is also a bad measurement to take, as you point out.

This gets into very complicated discussions. How do you measure productivity? How do you reward good performance? Can you use a reward for good (measured) performance as extrinsic motivation?

Anybody who tells you they have a good way of doing this that will work across a wide range of different tasks is either deluded or lying (or both). Lot's of successful businesses are run by deluded/lying people.