Hi Stephen,
That's a good questions, but it isn't metrics exactly in the way you're thinking of it.
Metrics tell what and how you are going to measure your product’s quality. They give you some objective measures to help you make better judgments about it. This is where you document how you’ll be figuring out the product’s quality. You need to write down the formulas you’ll use, when you will do the measurements, why you are taking them, and how you will interpret them.
Hope that helps.