Build waring introduced by gRPC example

[ThomsonTan]

For MSVC build with examples, a lot of protobuf warnings are emitted. We disabled all of warnings for opentelemetry-proto compilation, but the fix is not applied to gRPC example which also doesn't protobuf compilation.

Steps to reproduce
Build openetelemetry-cpp with WITH_OTLP with CMake.

What is the expected behavior?
The build should complete and clean.

What is the actual behavior?
A lot of warnings emitted.