Description:
Add support for parsing, interpreting, and serializing HTTP/2 METADATA frames. METADATA is an extension to the HTTP/2 protocol that will be used to convey information about request and response streams out-of-line.

Google will bring the draft METADATA specification to the IETF for standardization. If the specification is adopted and any changes are made as part of the process, we should update Envoy to match.

Relevant Links:
Rough draft METADATA specification: https://docs.google.com/document/d/1DsPjl6qWxhcF28vzJey0xQJ2O2eRKHv7CBXt3EZyDjc/edit?usp=sharing

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