Metric hacking is not growth hacking. Before it became "Growth Hacking", it was just creative, low-cost marketing with growth cycles (e.g. viral loops, etc.). At its core, it should move the needle in a positive, non-inflated way and as a result uncover profitable and sustainable strategies for growth. Period.

Metric hacking creates the facade of product market fit. This will hurt you in the long run. Choosing the right metrics can be difficult, but once you find it is will always be honest and guide you in the right direction.