Reconcile filter state and dynamic metadata

See started PR here as well as discussion: https://github.com/envoyproxy/envoy/pull/4873

At minimum we need clear docs on when to use each one. Optimally we would merge them. I have some thoughts on how to do this and can work on this if no one gets to it first.

cc @rshriram @htuch

Start by reading the discussion in PR #4873, which the issue identifies as the related work. Determine the intended relationship between filter state and dynamic metadata; done means documenting when to use each and, if the design is agreed, reconciling them rather than leaving the behavior split.

Written by the indexing model from the issue text.