R representation of Service Descriptors
TODO
pointer:External pointer to a google::protobuf::MethodDescriptor C++ object
name:fully qualified name of the method
service:fully qualified name of the service that defines this method
signature(x = "MethodDescriptor"): debug string of the method
signature(x = "MethodDescriptor"): debug string of the method
signature(x = "MethodDescriptor"): ...
signature(x = "MethodDescriptor"): ...
signature(object = "MethodDescriptor"): the '>Descriptor of the input type of the method
signature(object = "MethodDescriptor"): the '>Descriptor of the output type of the method