docs: async def _return_exported_sender(self, sender):
          """
          Returns a borrowed exported sender. If all borrows have
          been returned, the sender is cleanly disconnected.
          """
          async with self._borrow_sender_lock:
              dc_id = sender.dc_id
              n, _ = self._borrowed_senders[dc_id]
              n -= 1
              self._borrowed_senders[dc_id] = (n, sender)
              if not n:
                  self._log[__name__].info(
                      'Disconnecting borrowed sender for DC %d', dc_id)
                  await sender.disconnect()