How to record reverse proxy upstream server serving request in Nginx log?

First add new logging format

log_format upstreamlog '[$time_local] $remote_addr - $remote_user - $server_name $host to: $upstream_addr: $request $status upstream_response_time $upstream_response_time msec $msec request_time $request_time';

Example output:

[18/Nov/2019:10:08:15 -0700] <request IP> - - - <config host> <request host> to: 127.0.0.1:8000: GET /path/requested HTTP/1.1 200 upstream_response_time 0.000 msec 1574096895.474 request_time 0.001

and then redefine accesslog as

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

log_format goes to http {} section, access_log can be inside location.

Use $upstream_addr and you will get, for example, 127.0.0.1:8000 or unix:/home/my_user/www/my_site/tmp/.unicorn.sock