Separate OTLP proto from the transport like GRPC or HTTP?

[ThomsonTan]

The OTLP proto library is mostly used by the OTLP gRPC or HTTP exporter, but the proto library can also be used with other protocols, like in the user_events exporter.

Could we add a flash like WITH_OTLP_PROTO which builds the proto build, but don't enable the gRPC or HTTP exporter?

The current workaround used in the above user_events exporter is it enables OTLP/HTTP exporter as implicit dependence, even it doesn't use it really.