skfe - Record upstream_response_time in logs to help debug latency issues.

Bug: skia:
Change-Id: Id8728d17d18ae3cd913265e36757f45c6e5845aa
Reviewed-on: https://skia-review.googlesource.com/c/177326
Reviewed-by: Eric Boren <borenet@google.com>
Commit-Queue: Joe Gregorio <jcgregorio@google.com>
diff --git a/skfe/sys/skia_org_nginx b/skfe/sys/skia_org_nginx
index f5dba58..479e33b 100644
--- a/skfe/sys/skia_org_nginx
+++ b/skfe/sys/skia_org_nginx
@@ -15,7 +15,8 @@
 # Include the proxied host into the combined log format.
 log_format combined_proxy '$remote_addr - $remote_user [$time_local] '
                           '"$request" $status $body_bytes_sent '
-                          '"$http_referer" "$http_user_agent"'
+                          '"$http_referer" "$http_user_agent" '
+                          'upstream=$upstream_response_time '
                           '"$proxy_host"';
 
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