Make nginx to pass hostname of the upstream when reverseproxying

Actually you can do that via proxy_set_header.

For more details look here: http://nginx.org/en/docs/http/ngx_http_proxy_module.html#proxy_set_header or see an example use-case here: https://stackoverflow.com/questions/12847771/configure-nginx-with-proxy-pass

I have included the dynamic approach into your above posted configuration:

server {
  listen 80;
  server_name example.com;
  location / {
    proxy_pass       http://main;
    proxy_set_header Host            $host;
    proxy_set_header X-Forwarded-For $remote_addr;
  }
}

Here is an example with a static host name:

server {
  listen 80;
  server_name example.com;
  location / {
    proxy_pass       http://main;
    proxy_set_header Host            www.example.com;
    proxy_set_header X-Forwarded-For $remote_addr;
  }
}