Add support of additional %UPSTREAM% variables to custom headers

One of our customers is using the Envoy tap filter to retrieve transport information and add them to their request/response headers, as documented here : https://www.envoyproxy.io/docs/envoy/latest/configuration/http/http_conn_man/headers.html#custom-request-response-headers

At this point, very few %UPSTREAM% variable names are supported :
%UPSTREAM_METADATA([“namespace”, “key”, …])%
%UPSTREAM_REMOTE_ADDRESS%

They wish additional %UPSTREAM% variables were supported, such as %UPSTREAM_LOCAL_ADDRESS%.

Thanks !