cd nginx
./configure .... --add-module=../ngx_modules/ngx_http_prelog_module
make

Syntax:     prelog path;
            prelog path [format];
            prelog off;
Default:    off;
Context:    http, server, location

The directive solve the problem: when upstream block, nginx can't log into access_log, which make it hard to say who was the bad guy. prelog can log any request before dive into upstream. In another word, it log into path when a request go through the and of NGX_ACCESS_PHASE. Keep in mind that it only logging for the first location, if internal redirect existed, it not logging.

If format not given, it will use combined.

server {
    prelog /var/log/nginx/prelog.log main;

    location /test {
        prelog /var/log/nginx/prelog.log main;
    }
    
    location / {
        prelog /var/log/nginx/prelog.log;
    }

}