> in this situation is to give the manager some form of metric to describe the problem

how do you have a metric to measure a future issue that got prevented by having good maintenance?

Either you cannot actually imagine nor describe the problem that was prevented, or you could just make something up which cannot be disproved nor falsifiable. So if you asked for time/resources/budget to do maintenance, you cannot then give proof that this maintenance was useful!

The only way to get a metric is to have an incident or have issues crop up, and then in the retrospective, claim that certain maintenance work could've prevented it.