Can not build with `OTELCPP_MAINTAINER_MODE=ON` and examples

**Describe your environment**

Compiler: Clang 20.1.3
OS: Linux

```bash
/data/home/owentou/workspace/github/owent/opentelemetry-cpp/examples/otlp/http_instrumented_main.cc:61:12: error: unused variable 'serialize' [-Werror,-Wunused-variable]
   61 | std::mutex serialize;
      |            ^~~~~~~~~
5 warnings and 1 error generated.
```