Feature request: ability to use x-request-id in response_headers_to_add

[ascheglov]

Currently there is no way to use request headers in response_headers_to_add (it doesn't support %REQ(header-name)%).

However it would be nice to have an access to certain headers, namely x-request-id.

Normally a backend would echo the x-request-id header, so that it would be proxied to a client.
When there is a request timeout, envoy sends the "504 upstream request timeout" reply, and the client doesn't get the x-request-id.

Although it might be useful to echo other envoy-generated request headers (e.g. uber-trace-id for traces), for now it would be sufficient to add the %REQUEST_ID% in StreamInfoHeaderFormatter, requestId in StreamInfo, and set it in the http connection manager together with setDownstreamRemoteAddress, after the ConnectionManagerUtility::mutateRequestHeaders call.

[ascheglov]

And there is another thing, on request timeout, the Filter::onUpstreamAbort function calls sendLocalReply and it doesn't call finalizeResponseHeaders in its headers callback, so response_headers_to_add are not used anyways.

So there are two things to do:

1. add %REQ(x-request-id)% to response_headers_to_add
2. add a finalizeResponseHeaders call to onUpstreamAbort