def call_signature(request_method)
      desc = rpc_desc(request_method)
      desc&.name ? desc.name.to_s.underscore.to_sym : nil
    end
def call_signature(request_method)
      desc = rpc_desc(request_method)
      desc&.name ? desc.name.to_s.underscore.to_sym : nil
    end
Properly find the appropriate call signature for the GRPC::GenericService given the request method name

 @return [Symbol]