code: public static String getMapFieldKeyType(Field field) {
          FieldType type = field.getType();
          if (!(type instanceof Message)) {
              throw new IllegalArgumentException(field.toString());
          }
          Message entryType = (Message) type;
          ScalarFieldType keyType = (ScalarFieldType) entryType.getField(MAP_ENTRY_KEY).getType();
          return ScalarFieldTypeUtil.getWrapperType(keyType);
      }
docs: Returns map field key type name.