update issue template to encourage collaboration and metrics

.gitlab/issue_templates/Feature Proposal.md

-### Description
+### Problem to solve
 
-(Include problem, use cases, benefits, and/or goals)
+### Further details
+
+(Include use cases, benefits, and/or goals)
 
 ### Proposal
 
+### What does success look like, and how can we measure that?
+
+(If no way to measure success, link to an issue that will implement a way to measure this)
+
 ### Links / references
 
 /label ~"feature proposal"