Insights Gathered:
Collect metrics, catching the users completing a specific CUJ. If the CUJs are well mapped to UI flows, you can launch one survey per flow. If the CUJs are more freeform, ask the user which CUJ they are on in a multiple choice question. Then assess the metric you are interested in.
The metric should target the most important aspect of your product. It could be as simple and generic as Satisfaction or more specialized as ease of use or speed or whatever else you are trying to optimize.
Remember to include an open-ended follow-up question that asks the user to specify the reason for their rating. This makes the metrics actionable!