Enable request rate limiting using authenticated user

[geonus]

Add a new variable, say $krb_remote_user, similar to $remote_user, which could be used as a key for request rate limiting in nginx configuration.
"
The $remote_user variable is extracted by nginx from the
Authorization header only when using Basic authentication. The
SPNEGO auth module tries to make it work by providing a fake
"Authorization: Basic ..." header to nginx, but this won't work
for limit_req because rate limiting happens before access checks
(and so before the SPNEGO auth module adds the fake header).
... solutions include adding its own variable to the
module, so it can be used at any time (much like $remote_user when
using Basic authentication)...
"
(http://mailman.nginx.org/pipermail/nginx/2020-August/059756.html)