At the end of last year we introduced improved metrics custom formula editor. Since then we made some improvements for metrics UI and infrastructure. Here's a brief overview of the changes.
Metrics UI Improvements
Support For Non-process Entities
Now non-process entities like Team Iteration, Team, Program, User, Requester are supported by the Metrics Feature. For Custom Formula Metric all of these entities are supported. For Effort Via Relation only Team Iterations are supported as only they have ability to calculate efforts.