Add X-Request-Start header

To measure http request queue time.

Author: zacblazic

config/http.conf

@@ -77,6 +77,7 @@ http {
   proxy_set_header    X-Forwarded-Port    $proxy_x_forwarded_port;
   proxy_set_header    X-Request-ID        $proxy_x_request_id;
   proxy_set_header    X-Forwarded-Host    $proxy_x_forwarded_host;
+  proxy_set_header    X-Request-Start     "t=${msec}";
 
   # Latency headers
   add_header    X-Proxy-Backend-Connect-Time    $upstream_connect_time;