Commit mutations to the database.

        :rtype: datetime
        :returns: timestamp of the committed changes.

def commit(self):
        """Commit mutations to the database.

        :rtype: datetime
        :returns: timestamp of the committed changes.
        """
        self._check_state()
        database = self._session._database
        api = database.spanner_api
        metadata = _metadata_with_prefix(database.name)
        txn_options = TransactionOptions(read_write=TransactionOptions.ReadWrite())
        response = api.commit(
            self._session.name,
            self._mutations,
            single_use_transaction=txn_options,
            metadata=metadata,
        )
        self.committed = _pb_timestamp_to_datetime(response.commit_timestamp)
        return self.committed