Understaing kong access log - - [24/Jan/2018:19:55:50 +0000] “POST http://path HTTP/1.1” 200 747 “-” "-

what does 747 represent in kong access log?
is it request_time or byte sent?

By default Nginx uses combined:

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

So it looks to be $body_bytes_sent:

How to add total response time in access.log?

The only way to do it by now is using custom Nginx configuration:

How to add host variable in access log?