code: private static void checkType(
        com.netflix.servo.annotations.Monitor anno, Class<?> type, Class<?> container) {
      if (!isNumericType(type)) {
        final String msg = "annotation of type " + anno.type().name() + " can only be used"
            + " with numeric values, " + anno.name() + " in class " + container.getName()
            + " is applied to a field or method of type " + type.getName();
        throw new IllegalArgumentException(msg);
      }
    }
docs: Verify that the type for the annotated field is numeric.