code: public function getSourceColumnNames(GetPropertyOptionsEvent $event)
      {
          if (($event->getPropertyName() !== 'tag_srcsorting')
              || ($event->getEnvironment()->getDataDefinition()->getName() !== 'tl_metamodel_attribute')) {
              return;
          }
  
          $model = $event->getModel();
          $table = $model->getProperty('select_srctable');
  
          if (!$table || !$this->connection->getSchemaManager()->tablesExist([$table])) {
              return;
          }
  
          $result = [];
  
          $indexes = $this->connection->getSchemaManager()->listTableIndexes($table);
          foreach ($this->connection->getSchemaManager()->listTableColumns($table) as $column) {
              if (\array_key_exists($column->getName(), $indexes)) {
                  continue;
              }
              $colName = $column->getName();
  
              $result[$colName] = $colName;
          }
  
          $event->setOptions($result);
      }
docs: Retrieve all column names of type int for the current selected table.
  
  @param GetPropertyOptionsEvent $event The event.
  
  @return void