brpc支持获取grpc客户端ClientContext->AddMetadata()设置的信息

**Is your feature request related to a problem? (你需要的功能是否与某个问题有关?)**
我的服务混合使用brpc和grpc，grpc中有ClientContext和AddMetadata()来添加自定义元数据的功能，brpc中有相对应的功能吗？ 我在grpc中添加了metadata，在brpc中怎么取出来？ 或者反过来，我在brpc中设置元数据在grpc中怎么获取？

**Describe the solution you'd like (描述你期望的解决方法)**
我是想在rpc客户端设置元数据，在服务端读取。 有没有一种方式能使得在brpc和grpc上都生效。

**Describe alternatives you've considered (描述你想到的折衷方案)**

**Additional context/screenshots (更多上下文/截图)**

Start by comparing grpc ClientContext::AddMetadata() with brpc's existing client and server metadata handling, then trace how metadata crosses between the two RPC frameworks. Done means metadata set by a client in either framework can be read by the server in the other, with compatibility coverage for both directions.

Written by the indexing model from the issue text.