Fix health checks not working behind load balancers
The change in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24199 caused requests coming from a load balancer to arrive as 127.0.0.1 instead of the actual IP.
Rack::Request#ip behaves slightly differently different than
ActionDispatch::Request#remote_ip: the former will return the first
X-Forwarded-For IP if all of the IPs are trusted proxies, while the
second one filters out all proxies and falls back to REMOTE_ADDR, which
is 127.0.0.1.
For now, we can revert back to using Rack::Request because these
middlewares don't manipulate parameters. The actual fix problem involves
fixing Rails: https://github.com/rails/rails/issues/28436.
In the future, we might consider using the HTTP_X_REAL_IP header as
Sentry does: https://github.com/getsentry/raven-ruby/pull/546/files