c++与java的baidu_rpc_meta proto定义不一致

这是brpc c++版本中的RpcRequestMeta proto定义
https://github.com/apache/incubator-brpc/blob/master/src/brpc/policy/baidu_rpc_meta.proto

这是brpc java版本的RpcRequestMeta proto定义
https://github.com/baidu/brpc-java/blob/master/brpc-java-communication/src/main/proto/baidu_rpc_meta.proto

对于RpcRequestMeta中field 7的定义，c++里是optional string request_id, java里是repeated RpcRequestMetaExtField ext_fields

跨语言的client/server通信中可能会出现兼容性问题。各语言间使用的proto没有统一的管理？
@jamesge @wenweihu86

Read src/brpc/policy/baidu_rpc_meta.proto and brpc-java-communication/src/main/proto/baidu_rpc_meta.proto first. Compare field 7 and check the cross-language compatibility expectations; the issue needs a maintainer decision on the canonical definition and validation path before a newcomer can know what “done” means.

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