code: @Nullable
    public static Descriptor getMapValueMessageType(FieldDescriptor mapField) {
      FieldDescriptor valueDesc = mapField.getMessageType().findFieldByName("value");
      if (valueDesc.getType() == FieldDescriptor.Type.MESSAGE) {
        return valueDesc.getMessageType();
      } else {
        return null;
      }
    }
docs: Returns the descriptor representing the type of the value of the map field. Returns null if the
  map value isn't a message.