session - Session for DB operations.
        saving_classes - class objects that need to be saved e.g. Issue, Run
        item_counts - map from class name to the number of items, for preallocating
        id ranges

def reserve(
        self,
        session: Session,
        saving_classes: List[Type],
        item_counts: Optional[Dict[str, int]] = None,
        use_lock: bool = False,
    ) -> "PrimaryKeyGenerator":
        """
        session - Session for DB operations.
        saving_classes - class objects that need to be saved e.g. Issue, Run
        item_counts - map from class name to the number of items, for preallocating
        id ranges
        """
        query_classes = {cls for cls in saving_classes if cls in self.QUERY_CLASSES}
        for cls in query_classes:
            if item_counts and cls.__name__ in item_counts:
                count = item_counts[cls.__name__]
            else:
                count = 1
            self._reserve_id_range(session, cls, count, use_lock)

        return self