update {cn|en}/bvar.md

docs/cn/bvar.md

@@ -92,4 +92,4 @@ process_username : "gejun"
 
 # bvar导出到其它监控系统格式
 
-bvar已支持的其它监控系统格式有[Prometheus](https://prometheus.io)。将Prometheus的抓取url地址的路径设置为`/metrics`即可，例如brpc server跑在本机的8080端口，则抓取url配置为`127.0.0.1:8080/metrics`。
+bvar已支持的其它监控系统格式有[Prometheus](https://prometheus.io)。通过-prometheus_metrics_path可设置Prometheus的抓取url，默认路径为`/metrics`，例如brpc server跑在本机的8080端口，则在默认配置下抓取url为`127.0.0.1:8080/metrics`。

docs/en/bvar.md

@@ -92,4 +92,4 @@ The monitoring system should combine data on every single machine periodically a
 
 # Dump to the format of other monitoring system
 
-Currently monitoring system supported by bvar is [Prometheus](https://prometheus.io). All you need to do is to set the path in scraping target url to `/metrics`. For example, if brpc server is running in localhost on port 8080, the scraping target should be `127.0.0.1:8080/metrics`.
+Currently monitoring system supported by bvar is [Prometheus](https://prometheus.io). Flag -prometheus_metrics_path can be used to set the path of scraping target, and its default value is `/metrics`. For example, if brpc server is running on localhost:8080, the scraping target should be `127.0.0.1:8080/metrics` under default configuration.