body_bytes_sent always 0

[steveh]

Ok I've figured this out but I'm not sure if its a bug or a "feature"
the issue is that in stage #2 I was also setting:
proxy_set_header X-Bytes-Sent $body_bytes_sent;

This appears to cause $body_bytes_sent to be evaluated and "stored" so
that subsequent uses of $body_bytes_sent don't call the method
ngx_http_variable_body_bytes_sent to determine the amount of body bytes
sent but instead uses the already stored value, which is now out of
date.

In this case the solution is to not set X-Bytes-Sent to $body_bytes_sent
in previous phase of the request, however there may be other cases where
this is not possible, and where dynamic variable is required instead of
a static one.

Looking through the source the fix for this could be to set
NGX_HTTP_VAR_NOCACHEABLE for body_bytes_sent?

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,185270,185284#msg-185284