docs: def _add_column(self, label, field):
          """ Add a new column to the table. It will have the header
          text ``label``, but for data inserts and queries, the
          ``field`` name must be used. If necessary, this will expand
          the size of the sheet itself to allow for the new column. """
          # Don't call this directly.
          assert self.headers is not None
          cols = 0
          if len(self._headers) > 0:
              cols = max([int(c.cell.col) for c in self._headers])
          new_col = cols + 1
          if int(self._ws.col_count.text) < new_col:
              self._ws.col_count.text = str(new_col)
              self._update_metadata()
  
          cell = self._service.UpdateCell(1, new_col, label,
                                          self._ss.id, self.id)
          self._headers.append(cell)