One important thing that you need to keep in mind when defining UX metrics is that if you want to define metrics that really impact the product, they should not only think about improvements in the user experience, but they should also be linked to other product objectives. So, it's nice to think of three dimensions:
- Descriptive metrics, to help describe what happened
- Perception metrics, to focus on how customers perceive the product
- Result metrics, to help describe what customers have done or expect to do based on their perceptions
It is also important to redeem design principles, personas, and everything the product has from user information, and that can help define UX metrics.
In other words, defining metrics is ...