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

envoy/api/v2/core/protocol.proto

@@ -79,6 +79,14 @@ message Http2ProtocolOptions {
 
   // Allows proxying Websocket and other upgrades over H2 connect.
   bool allow_connect = 5;
+
+  // [#not-implemented-hide:] Hiding until envoy has full metadata support.
+  // Still under implementation. DO NOT USE.
+  //
+  // Allows metadata. See [metadata
+  // docs](https://github.com/envoyproxy/envoy/blob/master/source/docs/h2_metadata.md) for more
+  // information.
+  bool allow_metadata = 6;
 }
 
 // [#not-implemented-hide:]