Proxy metadata in responses (#5021)
Proxy metadata in responses (#5021)

Description: Allow envoy to proxy metadata in responses.

Risk Level: Low. Not used.
Testing: Integration test. (Unit tests will be added with filters)
Docs Changes: inline
Release Notes: n/a
Part of #2394

Signed-off-by: Yang Song <yasong@google.com>

Mirrored from https://github.com/envoyproxy/envoy @ b6ff832b9159fad4aef3e93d6243753f9427494c

lyft/envoy-api,lyft/envoy-api,lyft/envoy-api