Set header $upstream_response_time with proxy_cache directive

[Dewangga Bachrul Alam]

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Hello!

I have problem how to debugs current response time to upstream, my
configuration is looks likes :

...
upstream upstream_distribution {
        server full-fqdn.tld;
}

# common configuration

        location ~ \.(jpe?g|png|gif|webp)$ {
                proxy_pass      http://upstream_distribution;
                proxy_cache_use_stale error timeout updating http_500
http_502 http_503 http_504;
                proxy_cache_valid 200 302 301 3d;
                proxy_cache     images;
                proxy_cache_valid any 3s;
                proxy_cache_lock on;
                proxy_cache_min_uses 1;
                proxy_ignore_headers Cache-Control Expires;
                proxy_hide_header X-Cache;
                proxy_hide_header Via;
                proxy_hide_header ETag;
                more_set_headers "X-Cache: $upstream_cache_status";
		more_set_headers "X-RTT: $upstream_response_time ms";
        }

# common configuration
...

The problem is, the request was cached by nginx and didn't send
response to upstream, so the response of the header was :

age:94
cache-control:max-age=604800, public
content-length:143862
content-type:image/webp
date:Fri, 04 Nov 2016 09:03:44 GMT
last-modified:Fri, 04 Nov 2016 05:41:55 GMT
x-rtt:ms
server:nginx
status:200
x-cache:HIT

Is it possible to capture upstream_response_time while the content was
cached?

Any hints and helps are appreciated.
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