baggage.h is missing #include <cctype> for MSVC2015

[lidavidm]

Describe your environment

Windows 10 x64
MSVC 19.00.24210 (VS2015)

Steps to reproduce

OpenTelemetry (latest main @ 12e56f9) was built with cmake -GNinja .. -DWITH_API_ONLY=ON -DCMAKE_INSTALL_PREFIX=../install -DBUILD_TESTING=OFF.

Then I tried to build a trivial application including an OpenTelemetry header:

#include <opentelemetry/baggage/baggage.h>

int main(int, char**) {
    return 0;
}

What is the expected behavior?
The application should build successfully.

What is the actual behavior?

C:\Users\User\opentelemetry-cpp\install>cl.exe -Iinclude /EHsc /DHAVE_ABSEIL_VARIANT=1 test.cc
Microsoft (R) C/C++ Optimizing Compiler Version 19.00.24210 for x64
Copyright (C) Microsoft Corporation.  All rights reserved.

test.cc
include\opentelemetry/baggage/baggage.h(225): error C2039: 'isalnum': is not a member of 'std'
c:\users\user\opentelemetry-cpp\install\include\opentelemetry\nostd\./absl/types/variant.h(809): note: see declaration of 'std'
include\opentelemetry/baggage/baggage.h(248): error C2039: 'isdigit': is not a member of 'std'
c:\users\user\opentelemetry-cpp\install\include\opentelemetry\nostd\./absl/types/variant.h(809): note: see declaration of 'std'
include\opentelemetry/baggage/baggage.h(252): error C2039: 'isdigit': is not a member of 'std'
c:\users\user\opentelemetry-cpp\install\include\opentelemetry\nostd\./absl/types/variant.h(809): note: see declaration of 'std'
include\opentelemetry/baggage/baggage.h(252): error C2039: 'toupper': is not a member of 'std'
c:\users\user\opentelemetry-cpp\install\include\opentelemetry\nostd\./absl/types/variant.h(809): note: see declaration of 'std'
include\opentelemetry/baggage/baggage.h(273): error C2039: 'isalnum': is not a member of 'std'
c:\users\user\opentelemetry-cpp\install\include\opentelemetry\nostd\./absl/types/variant.h(809): note: see declaration of 'std'

Additional context

Adding #include <cctype> to baggage.h fixes the issue.

[ThomsonTan]

@lidavidm thanks for reporting and investigation. Would you like to submit a PR with the fix?

[maxgolov]

If you don't mind, could you please apply the fix, and try pulling locally from this PR:
#771

This PR eliminates the HAVE_ABSEIL_VARIANT option. Making Abseil Variant now the default option. That way we can build with MSVC2015 and the binary is going to be compatible with 2017 and 2019 (and hopefully 2022). I think we also need to add unit tests for baggage - to catch issues like this. Surprised we didn't catch it in my other PR. I'm planning to add CI loop for vs2015 once the above PR is merged.

[lidavidm]

Sorry about the delay - I'll submit a PR and test against the Abseil branch when I get a chance.