variable 'nremoved' set but not used warnings when compile master using clang-14

[thorneliu]

Describe the bug (描述bug)

/Users/apple/Documents/brpc/src/bvar/detail/sampler.cpp:163:13: warning: variable 'nremoved' set but not used [-Wunused-but-set-variable]
        int nremoved = 0;
            ^
/Users/apple/Documents/brpc/src/bvar/detail/sampler.cpp:164:13: warning: variable 'nsampled' set but not used [-Wunused-but-set-variable]
        int nsampled = 0;
/Users/apple/Documents/brpc/src/brpc/details/hpack.cpp:492:12: warning: variable 'out_bytes' set but not used [-Wunused-but-set-variable]
    size_t out_bytes = 1;
Users/apple/Documents/brpc/src/brpc/span.cpp:752:17: warning: variable 'i' set but not used [-Wunused-but-set-variable]
    for (size_t i = 0; nscan < max_scan && it->Valid(); ++i, it->Prev()) {
                ^

To Reproduce (复现方法)
compile

mkdir build
cd build
cmake ../
make -j3

Expected behavior (期望行为)
No warnings

Versions (各种版本)
OS: MacOS Ventura 13.3.1
Apple clang version 14.0.3 (clang-1403.0.22.14.1)
Target: x86_64-apple-darwin22.4.0
Thread model: posix
InstalledDir: /Library/Developer/CommandLineTools/usr/binCompiler:

brpc:
5cc470b

protobuf:
3.31.12

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