Hello,

we would like to customize the response code of upstream reset. Right now, we see that 503 is hardcoded in Filter::onUpstreamReset

Motivation

Status code
Without Envoy, when app A was communicating with B and was not available, the A's client received connection refused. With Envoy in place, the A will receive 503 status code. We would like to differentiate this behaviour with different status code, for example 504 or 502, so our developers that this is not a problem with B itself throwing 503, but with a connection to B.

If you think that makes sense, we are ready to contribute it, but need a guidance where to put a parameters for this.