add Prometheus docs

docs/cn/bvar.md

@@ -89,3 +89,7 @@ process_username : "gejun"
 ![img](../images/bvar_noah2.png)
 
 ![img](../images/bvar_noah3.png)
+
+# 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`。

docs/en/bvar.md

@@ -89,3 +89,7 @@ The monitoring system should combine data on every single machine periodically a
 ![img](../images/bvar_noah2.png)
 
 ![img](../images/bvar_noah3.png)
+
+# 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`.