Add `$request_time` to nginx logs

In order to get more detail on what is causing speed issues with monitor.gitlab.net, it would be useful to have $request_time0 added to the nginx logs.

My proposed change would be to adjust the default combined log format to include this extra field.

log_format combined_time '$remote_addr - $remote_user [$time_local] '
                         '"$request" $status $body_bytes_sent '
                         '"$http_referer" "$http_user_agent" '
                         '$request_time';

access_log /var/log/nginx/access.log combined_time;