[WIP] Adjust the standards for proto target to align with those of Protocol Buffers.

[owent]

Fixes #3799

Changes

Please provide a brief description of the changes here.

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

Analysis

The generated .pb.cc codes

inline constexpr EntityRef::Impl_::Impl_(
    ::_pbi::ConstantInitialized) noexcept
      : id_keys_{},
        description_keys_{},
        schema_url_(
            &::google::protobuf::internal::fixed_address_empty_string,
            ::_pbi::ConstantInitialized()),
        type_(
            &::google::protobuf::internal::fixed_address_empty_string,
            ::_pbi::ConstantInitialized()),
        _cached_size_{0} {}

port.h in protobuf:

// Take advantage of C++20 constexpr support in std::string.
class alignas(8) GlobalEmptyStringConstexpr {
 public:
  const std::string& get() const { return value_; }
  // Nothing to init, or destroy.
  std::string* Init() const { return nullptr; }

  template <typename T = std::string, bool = (T(), true)>
  static constexpr std::true_type HasConstexprDefaultConstructor(int) {
    return {};
  }
  static constexpr std::false_type HasConstexprDefaultConstructor(char) {
    return {};
  }

 private:
  std::string value_;
};

using GlobalEmptyString = std::conditional_t<
    GlobalEmptyStringConstexpr::HasConstexprDefaultConstructor(0),
    const GlobalEmptyStringConstexpr, GlobalEmptyStringDynamicInit>;

PROTOBUF_EXPORT extern GlobalEmptyString fixed_address_empty_string;

port.cc in protobuf

PROTOBUF_ATTRIBUTE_NO_DESTROY PROTOBUF_CONSTINIT
    PROTOBUF_ATTRIBUTE_INIT_PRIORITY1 GlobalEmptyString
        fixed_address_empty_string{};

If we build protobuf with C++17, but build generated .pb.cc with C++20, protobuf will only export GlobalEmptyStringDynamicInit fixed_address_empty_string; while .pb.cc will use GlobalEmptyStringConstexpr fixed_address_empty_string;, which will cause unresolved external symbol.

[Copilot]

Pull request overview

This PR adds CI validation to ensure OpenTelemetry C++ can be built with a different C++ standard than the protobuf dependency it uses, addressing issue #3799 where MSVC linking errors occurred when protobuf was compiled with C++17 but OpenTelemetry was built with C++20.

Key Changes:

• Adds new cmake.different_std.test CI target for both Linux (GCC) and Windows (MSVC) platforms
• Linux GCC test installs dependencies with C++17, then builds OpenTelemetry with C++23
• Windows MSVC test validates similar scenario with C++20 for OpenTelemetry

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
ci/do_ci.sh	Adds new cmake.different_std.test target for Linux/GCC that builds with C++23 STL and enables OTLP file support
ci/do_ci.ps1	Adds new cmake.different_std.test target for Windows/MSVC that builds with C++20 STL, maintainer mode, and OTLP file support
.github/workflows/ci.yml	Adds two new CI jobs: one for GCC (installs protobuf with C++17, builds with C++23) and one for MSVC (tests different C++ standards)

[Copilot]

The PowerShell script sets DWITH_STL=CXX20 (line 278) and also explicitly sets DCMAKE_CXX_STANDARD=20 (line 279). However, the bash script only sets DWITH_STL=CXX23 (line 343) without explicitly setting CMAKE_CXX_STANDARD. This inconsistency could lead to different behaviors. Consider either adding CMAKE_CXX_STANDARD to the bash script or removing it from the PowerShell script to maintain consistency, or align both to use the same C++ standard version.

Suggested change

	-DWITH_ASYNC_EXPORT_PREVIEW=ON \
	-DWITH_ASYNC_EXPORT_PREVIEW=ON \
	-DCMAKE_CXX_STANDARD=23 \

[owent]

The environment already set CMAKE_CXX_STANDARD, it should not be set here again.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.93%. Comparing base (f33dcc0) to head (7cab857).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3800      +/-   ##
==========================================
+ Coverage   89.91%   89.93%   +0.02%     
==========================================
  Files         225      225              
  Lines        7163     7163              
==========================================
+ Hits         6440     6441       +1     
+ Misses        723      722       -1     

see 1 file with indirect coverage changes

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