docs: public static void validateMethodOrThrow(Method method) {
      Class<?> returnType = method.getReturnType();
      if (returnType.equals(Void.TYPE)) {
        return;
      } else if (!Publisher.class.isAssignableFrom(returnType)) {
        throw new UnsupportedOperationException("Service method return type can be Publisher only");
      }
      if (method.getParameterCount() > 1) {
        throw new UnsupportedOperationException("Service method can accept 0 or 1 parameters only");
      }
    }