There is actually a term for this in economics. It's called the "Lucas critique", and effectively states that any metric will be gamed eventually. It's slightly more subtle and is actually a critique about macroeconomic policymaking -- that people's actions are not structural and that making a new policy will affect how people act as they try to (ab)use the new policy -- but that's the basic gist.

EDIT: To clarify -- I'm not an economist, it's just something I read about in my spare time. I'm sure that the Lucas critique was pre-dated by other ideas (it's far too obvious to be entirely novel), and that there's a much deeper analysis that I wasn't aware of.