nginx logging depending on upstream server

nginx.conf

upstream {
  server 10.0.0.200;
  server 10.0.0.300;
}

map $upstream_addr $should_log {
  "10.0.0.200:80" 1;
  default 0;
}

server {
  ...
  access_log /var/log/nginx/single_upstream_host.log if=$should_log;
  ...
}