Synthetic proxy responses should contain an identifying header

Nobody has claimed this yet.

All responses returned by the proxy are either a proxied response from another server or a synthetic response generated by the proxy (e.g. if the proxy encounters an error and returns a 500 or if the proxy times out a request and returns a 504).

When debugging issues, it can be difficult to know for sure if a response (especially a 500 response with no body) was generated by Linkerd or by a service that Linkerd is proxying to. If the proxy always adds some kind of identifying header on all synthetic responses, it would make this clear.

The inverse of this is having Linkerd add the Via header to responses that it proxies and this can be useful as well, especially if it is unclear whether a request is transitting a proxy or not (such as when debugging iptables issues).

Start by tracing Linkerd's proxy response-generation entry points for synthetic 500 and 504 responses; the issue does not name specific files or tests. Determine how all synthetic responses can be distinguished from proxied responses, then add coverage showing the identifying header is present on each synthetic response.

Written by the indexing model from the issue text.