Span::CreateClientSpan是否有线程安全问题

**Describe the bug (描述bug)**

https://github.com/apache/brpc/blob/master/src/brpc/span.cpp#L133

在代码这个位置，如果是在多个bthread中调用的Span::CreateClientSpan，而他们属于同一个Span的child span。此时他们对brpc::Span::_next_client是存在竞态的。
即便认为这个指针的赋值是原子的，似乎也可能导致内存泄漏。

这里是有什么特殊的设计避免了竞态吗？求解答

**To Reproduce (复现方法)**

**Expected behavior (期望行为)**

**Versions (各种版本)**
OS:
Compiler:
brpc:
protobuf:

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

Start at src/brpc/span.cpp#L133 and trace how concurrent bthreads call Span::CreateClientSpan when sharing a parent Span. Inspect accesses to brpc::Span::_next_client, then determine whether the pointer update can race or leak memory. Done means the thread-safety question is resolved with evidence from the implementation and a focused concurrency reproduction.

Written by the indexing model from the issue text.