A regular (non-regex) file: upstream needs to strip the prefix so that it is equivalent to "mounting" the specified directory under the configured path in the URL space, but with regex rewriting the target path is determined by the rewriteTarget.
Fixesoauth2-proxy/oauth2-proxy#2242
* Use a specialized ResponseWriter in middleware
* Track User & Upstream in RequestScope
* Wrap responses in our custom ResponseWriter
* Add tests for logging middleware
* Inject upstream metadata into request scope
* Use custom ResponseWriter only in logging middleware
* Assume RequestScope is never nil