Log the headers sent by nginx to upstream server

[Sushma]

My client sends a request to nginx with a http header set. 
This header is overwritten by nginx before passing along the reqest to the
downstream server.
I want to log the header after modification by nginx.
I understand that we can log the header sent by client using
"$http_<header_name>".
Is there some embedded variable or some other way I can use which will log
the header sent to the downstream server?

Posted at Nginx Forum: https://forum.nginx.org/read.php?2,269300,269300#msg-269300