Compiling severity.h with gcc-15 fails

[AngryLoki]

Describe your environment

Reported originally in https://bugs.gentoo.org/946146 for opentelemetry-cpp-1.16.1, but still reproducible with 1.18.0.

Build with gcc-15 fails with:

/var/tmp/portage/dev-cpp/opentelemetry-cpp-1.16.1/work/opentelemetry-cpp-1.16.1/api/include/opentelemetry/logs/severity.h:49:7: error: ‘uint8_t’ does not name a type
   49 | const uint8_t kMaxSeverity = 255;
      |       ^~~~~~~

With some other gcc/clang versions it worked due to transitive includes, but it was very unpredictable (for example is available after inclusion if string_view with libc++, or ostream with libstdc++, but not for gcc-13). Here is a conformance view if you are interested: https://godbolt.org/z/hEs18z8Pd

See linked pull-request with a fix.