What kind of metrics, if any, can be collected from requirements development?

Question

We elicit requirements from our product stakeholders by creating and refining user stories in monthly sprint planning meetings. This includes defining use cases, acceptance criteria, and identifying constraints, and the final result is a sprint backlog of user stories. The high-level customer requirements are refined into lower-level product requirements through design reviews among the development team throughout the sprint. The design reviews result in UML diagrams, software interfaces, and, in general, a human language description of various software requirements.

We are increasing our metric-awareness as an organization so that we can better identify potential areas of process improvement (and so that we can eventually be appraised at CMMI level 3).

What kind of useful metrics are there to collect in the elicitation and refinement of customer requirements?