Switch from MPark variant to absl::variant as default (#771)

CMakeLists.txt

@@ -53,15 +53,19 @@ if(WITH_ABSEIL)
   add_definitions(-DHAVE_ABSEIL)
   find_package(absl CONFIG REQUIRED)
 
-  # Abseil headers-only lib is needed for absl::variant to work vs2015.
-  # `nostd::mpark::variant` is not compiling in vs2015.
   set(CORE_RUNTIME_LIBS absl::any absl::base absl::bits absl::city)
 
   # target_link_libraries(main PRIVATE absl::any absl::base absl::bits
   # absl::city)
 endif()
 
 if(WITH_STL)
+  # These definitions are needed for test projects that do not link against
+  # opentelemetry-api library directly. We ensure that variant implementation
+  # (absl::variant or std::variant) in variant unit test code is consistent with
+  # the global project build definitions.
+  add_definitions(-DHAVE_CPP_STDLIB)
+  add_definitions(-DHAVE_GSL)
   # Require at least C++17. C++20 is needed to avoid gsl::span
   if(CMAKE_MINOR_VERSION VERSION_GREATER "3.18")
     # Ask for 20, may get anything below

api/include/opentelemetry/common/attribute_value.h

@@ -40,6 +40,7 @@ using AttributeValue =
                    int64_t,
                    uint32_t,
                    double,
+                   const char *,
                    nostd::string_view,
                    nostd::span<const bool>,
                    nostd::span<const int32_t>,
@@ -64,6 +65,7 @@ enum AttributeType
   kTypeInt64,
   kTypeUInt,
   kTypeDouble,
+  kTypeCString,
   kTypeString,
   kTypeSpanBool,
   kTypeSpanInt,

api/include/opentelemetry/nostd/absl/README.md

@@ -2,13 +2,3 @@
 
 This is a snapshot of Abseil Variant `absl::variant` from Abseil
 `v2020-03-03#8`.
-
-This code is required to compile OpenTelemetry code in vs2015 because MPark
-Variant implementation is not compatible with vs2015.
-
-Build option `HAVE_ABSEIL_VARIANT` allows to enable the build with
-`absl::variant`, `absl::get` and `absl::visit` as defalt implementation for
-`nostd::` classes.
-
-Going forward it makes sense to use `absl::variant` as default implementation
-for Windows OS and vs2015, vs2017, vs2019 and newer compilers.