[rpcz] 支持brpc stream的trace

**Is your feature request related to a problem? (你需要的功能是否与某个问题有关?)**
在我们的场景由于需要支持消息保序和性能的原因，需要使用brpc stream，基于IOBuf再封装了一层RPC，类似于grpc的stream模式。 但目前brpc的tracing对stream的支持比较欠缺，比如rpcz不支持trace streaming的消息

**Describe the solution you'd like (描述你期望的解决方法)**

**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 locating brpc's rpcz tracing entry points and the existing brpc stream handling around IOBuf-based messages. Trace how ordinary RPC messages reach rpcz, then determine the project’s expected representation for streaming messages; done means rpcz can trace brpc stream traffic while preserving the existing behavior for regular RPCs.

Written by the indexing model from the issue text.