nginx: log subrequests

roles/nginx/templates/nginx.conf.j2

@@ -14,6 +14,8 @@ http {
     include       /etc/nginx/mime.types;
     default_type  application/octet-stream;
     
+    log_subrequest on;
+
     log_format  main  '$remote_addr - $remote_user [$time_local] "$request" '
                       '$status $body_bytes_sent "$http_referer" '
                       '"$http_user_agent" "$http_x_forwarded_for"';