code: protected FieldDescriptor resolvePayloadField(Message message) {
          for (FieldDescriptor field : message.getDescriptorForType().getFields()) {
              if (message.hasField(field)) {
                  return field;
              }
          }
  
          throw new RuntimeException("No payload found in message " + message);
      }
docs: Find out which field in the incoming message contains the payload that is.
  delivered to the service method.