design/37112-unstable-runtime-metrics.md: add proposal

This change adds a proposal and design document for addition of a
runtime metrics package which is designed to support adding and removing
runtime metrics without breaking the Go 1 compatibility promise.

Updates golang/go#37112.

Change-Id: I550740970ad74c5c71d712735e4984808bdbf463
Reviewed-on: https://go-review.googlesource.com/c/proposal/+/218677
Reviewed-by: Austin Clements <[email protected]>

Author: mknyszek