Example to reproduce $upstream_response_time being a dash.

nginx.conf

events {}

http {
    log_format main '$upstream_addr $upstream_status $upstream_response_time';
    access_log /dev/stdout main;

    server {
        listen 8000;

        location / {
            proxy_pass  http://localhost:8001;

            add_header X-Trip-Time    $request_time;
            add_header X-Addr         $upstream_addr;
            add_header X-Status       $upstream_status;
            add_header X-Process-Time $upstream_response_time;
        }
    }
}