Add metrics form NGINX Plus API version 6

The [/stream/limit_conns/](https://nginx.org/en/docs/http/ngx_http_api_module.html#stream_limit_conns_), [/http/limit_conns/](https://nginx.org/en/docs/http/ngx_http_api_module.html#http_limit_conns_), and [/http/limit_reqs/](https://nginx.org/en/docs/http/ngx_http_api_module.html#http_limit_reqs_) data were added in version 6

