Missing fix for test for #834 and some examples have different definetions from `opentelemetry_api`

[owent]

Describe your environment
cmake 3.20.0
gcc 10.2.0
linux/centos 7

Steps to reproduce
Enable testing and install abseil with vcpkg.

What is the expected behavior?
Compiling success.

What is the actual behavior?

nostd::bad_variant_access undeclared in api/test/nostd/variant_test.cc:52:45 .

Additional context

After add using absl::bad_variant_access into api/include/opentelemetry/nostd/variant.h .
These messages show below.

FAILED: examples/plugin/load/CMakeFiles/load_plugin_example.dir/main.cc.o 
/data/prebuilt/gcc-10.2.0/bin/c++  -Igenerated/third_party/opentelemetry-proto -I../api/include -I../sdk/include -I../sdk -I../ext/include -O2 -g -DNDEBUG -std=gnu++11 -MD -MT examples/plugin/load/CMakeFiles/load_plugin_example.dir/main.cc.o -MF examples/plugin/load/CMakeFiles/load_plugin_example.dir/main.cc.o.d -o examples/plugin/load/CMakeFiles/load_plugin_example.dir/main.cc.o -c ../examples/plugin/load/main.cc
In file included from ../api/include/opentelemetry/common/attribute_value.h:10,
                 from ../api/include/opentelemetry/common/key_value_iterable.h:6,
                 from ../api/include/opentelemetry/plugin/tracer.h:8,
                 from ../api/include/opentelemetry/plugin/factory.h:9,
                 from ../api/include/opentelemetry/plugin/detail/dynamic_load_unix.h:12,
                 from ../api/include/opentelemetry/plugin/dynamic_load.h:11,
                 from ../examples/plugin/load/main.cc:4:
../api/include/opentelemetry/nostd/variant.h:62:13: 错误：‘class absl::otel_v1::bad_variant_access’ conflicts with a previous declaration

I think plugins should have the same definitions as opentelemetry_api.