design: add proposal document for profile labeling
design: add proposal document for profile labeling

Updates golang/go#17280

Change-Id: I6dcad5c98f6b19635bfce61d75e7052c05985131
Reviewed-on: https://go-review.googlesource.com/30015
Reviewed-by: Austin Clements <7ea35d812706d9213868749011af1ed4fa2f6aa0@google.com>

golang/proposal,golang/proposal