[BUILD] Build break using protoc 3.14

[marcalff]

[ 70%] Building CXX object exporters/otlp/CMakeFiles/otlp_recordable_test.dir/test/otlp_recordable_test.cc.o
In file included from /usr/include/gtest/gtest.h:62,
                 from /home/malff/CODE/MARC_GITHUB/opentelemetry-cpp/exporters/otlp/test/otlp_recordable_test.cc:4:
/home/malff/CODE/MARC_GITHUB/opentelemetry-cpp/exporters/otlp/test/otlp_recordable_test.cc: In member function ‘virtual void opentelemetry::v2::exporter::otlp::OtlpRecordable_SetInstrumentationScopeWithAttributes_Test::TestBody()’:
/home/malff/CODE/MARC_GITHUB/opentelemetry-cpp/exporters/otlp/test/otlp_recordable_test.cc:139:40: error: ‘const class opentelemetry::proto::common::v1::AnyValue’ has no member named ‘has_string_value’; did you mean ‘set_string_value’?
  139 |   ASSERT_TRUE(proto_attributes.value().has_string_value());
      |                                        ^~~~~~~~~~~~~~~~
make[2]: *** [exporters/otlp/CMakeFiles/otlp_recordable_test.dir/build.make:76: exporters/otlp/CMakeFiles/otlp_recordable_test.dir/test/otlp_recordable_test.cc.o] Error 1
make[1]: *** [CMakeFiles/Makefile2:6262: exporters/otlp/CMakeFiles/otlp_recordable_test.dir/all] Error 2
make: *** [Makefile:146: all] Error 2

The issue with this line:

ASSERT_TRUE(proto_attributes.value().has_string_value());

is that has_string_value() was added only in protoc 3.15, and this breaks with protoc 3.14.

• https://github.com/protocolbuffers/protobuf/releases/tag/v3.15.0

Now Proto3 Oneof fields have "has" methods for checking their presence in
C++.

The break can be seen also in CI in the include-what-you-use logs:

[ 76%] Building CXX object exporters/otlp/CMakeFiles/otlp_recordable_test.dir/test/otlp_recordable_test.cc.o
/home/runner/work/opentelemetry-cpp/opentelemetry-cpp/exporters/otlp/test/otlp_recordable_test.cc:139:40: error: no member named 'has_string_value' in 'opentelemetry::proto::common::v1::AnyValue'
  ASSERT_TRUE(proto_attributes.value().has_string_value());
              ~~~~~~~~~~~~~~~~~~~~~~~~ ^