[Exporter] Prevent infinite recursion with grpc

[marcalff]

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

• [OTel] Basic C++ OTel Stats Functionality grpc/grpc#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:

• Use Context to stop tracing opentelemetry-specification#530

[lalitb]

Thanks for raising the issue. Just to add the approach used by otel-python - https://github.com/search?q=repo%3Aopen-telemetry%2Fopentelemetry-python%20_SUPPRESS_INSTRUMENTATION_KEY%20&type=code

• Add the SUPPRESSION_KEY as true in the current active context - this can be done at the start on the Exporter::export() method, and set back to false at the end.
• Check for this key status from the context within the processor, and don't send the span to the exporter if set as true.

However, this approach has some constraint. Any instrumentation that needs to be suppressed in the libraries used by the OTLP exporter should be invoked from the exporter thread since the active context is thread-local in otel-cpp.

Similar approach used in otel-dotnet - https://github.com/open-telemetry/opentelemetry-dotnet/blob/1a836432840d657926923ccce4d09c4a4e08728c/src/OpenTelemetry/SuppressInstrumentationScope.cs#L12

[punya]

Here is a similar issue with a fix (OTel-C++ but a different gRPC-based exporter): googleapis/google-cloud-cpp#14611