[Exporter] Prevent infinite recursion with grpc

[marcalff]

The grpc C++ library is instrumented using opentelemetry-cpp:

• https://github.com/grpc/grpc/pull/33650

The opentelemetry-cpp OTLP GRPC exporter uses grpc

In the OTLP GRPC exporter implementation, opentelemetry-cpp needs to:

• disable instrumentation before making calls to grpc
• reenable instrumentation after grpc calls

so avoid infinite recursion.

This is blocking:

• linking opentelemetry-cpp with an instrumented grpc library is likely to cause issues.

This is blocked by the spec:

• https://github.com/open-telemetry/opentelemetry-specification/issues/530