$sent_http_* vars aren't correct when log_subrequest on

[n4nn31355]

Initially nginx logs the $sent_http_* variables for the targeted url:

nginx_1        | uri_path="/" uri_query="-" status="200" http_svc="handler1"

But when we use log_subrequest on, we get the subrequest headers in logs:

nginx_1        | uri_path="/__auth__" uri_query="-" status="200" http_svc="handler2"
nginx_1        | uri_path="/" uri_query="-" status="200" http_svc="handler2"

This problem also actual when we use these variables for tags.

There is a slightly modified jaeger example to represent the issue:
n4nn31355@bd63bb3