Rename agent to user_agent field in nginx log configuration

Author: mariusmitrofan

nginx.conf

@@ -13,7 +13,7 @@ http {
     vhost_traffic_status_zone shared:vhost_traffic_status:16m;
 
     log_format json escape=json '{'
-                    '"agent": "$http_user_agent", '
+                    '"user_agent": "$http_user_agent", '
                     '"body_bytes_sent": "$body_bytes_sent", '
                     '"bytes_sent": "$bytes_sent", '
                     '"clientip": "$remote_addr", '