rpcz does not record root client spans when Controller only keeps a weak_ptr to Span

[lh2debug-2]

Describe the bug

When rpcz is enabled in a brpc client process, root client-side RPC spans may not be recorded if the RPC is initiated without an existing local parent span.

The client process can successfully enable /rpcz, and rpcz internal service calls may appear, but application-level client RPC spans are missing from the rpcz output.

Root cause: Controller::_span is stored as std::weak_ptr<Span>. For a root client span, there is no local parent span and therefore no parent _client_list holding a strong reference. Once the caller-side temporary shared_ptr<Span> goes out of scope, the span can be destroyed before Controller::SubmitSpan() submits it to rpcz.

To Reproduce

1. Enable rpcz in a brpc client process.
2. Send an outgoing RPC from the client without an existing local parent span.
3. Check the client-side /rpcz output.
4. Observe that rpcz internal calls may be visible, but the application-level outgoing client RPC span is missing.

Expected behavior

Client-side root RPC spans should remain alive until the RPC finishes and Controller::SubmitSpan() has a chance to submit them to rpcz.

The outgoing client RPC should be visible in client-side /rpcz.

Versions

OS:

Compiler:

brpc:

protobuf:

Additional context/screenshots

Nested client spans created under a server span are less likely to hit this issue because the parent span owns child spans through _client_list.

A possible fix is to let Controller hold the current RPC span with std::shared_ptr<Span> until the RPC finishes, SubmitSpan() runs, or the Controller is reset. Child-to-parent references can remain weak, so this does not introduce a shared_ptr cycle.