Fix build with C++20.

[bavshin-f5]

OpenTelemetry SDK built with C++20 uses std::span in the API, and we want to match the standard to avoid type changes at the API boundary.

However, std::span constructor is explicit for non-default Extent, requiring us to use a more verbose specification instead of an initializer list.

src/trace_context.hpp:75:33: error: converting to ‘opentelemetry::v1::nostd::span<char, 32>’ {aka ‘std::span<char, 32>’} from initializer list would use explicit constructor ‘constexpr std::span<_Type, _Extent>::span(_It, size_type) [with _It = char*; _Type = char; long unsigned int _Extent = 32; size_type = long unsigned int]’
   75 |         tc.traceId.ToLowerBase16({out, kTraceIdSize});
      |         ~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~