chore: add comments for RouterMetaExtractor (#15)

README.md

@@ -101,6 +101,7 @@ In order to match the rules defined in the VirtualService we need to specify the
 For example: set the key and value to "stage" and "canary" to match the above rules defined in VirtualService.
 
 * We can first define a MetaExtractor and pass it to `RouterMiddleware` through `xdssuite.WithRouterMetaExtractor`.
+  * Notice: If RouterMetaExtractor is not configured, metainfo.GetAllValues will be used by default.
 ```
 var (
 	routeKey   = "stage"

README_CN.md

@@ -101,6 +101,7 @@ spec:
 比如：将 key 和 value 设置为“stage”和“canary”，以匹配 VirtualService 中定义的上述规则。
 
 * 我们可以先定义一个元信息提取方法，并通过 `xdssuite.WithRouterMetaExtractor` 传入到 `RouterMiddleware` 中。
+  * 注意：若未配置 RouteMetaExtractor，默认会使用 metainfo.GetAllValues 来获取。
 ```
 var (
 	routeKey   = "stage"

xdssuite/option.go

@@ -26,7 +26,7 @@ type routerMetaExtractor func(context.Context) map[string]string
 
 // Options for xds suite
 type Options struct {
-	routerMetaExtractor routerMetaExtractor
+	routerMetaExtractor routerMetaExtractor // use metainfo.GetAllValues by default.
 }
 
 func (o *Options) Apply(opts []Option) {