Lists the clients.

        :param Optional[str] instance: A Yamcs instance name.
        :rtype: ~collections.Iterable[yamcs.model.Client]

def list_clients(self, instance=None):
        """
        Lists the clients.

        :param Optional[str] instance: A Yamcs instance name.
        :rtype: ~collections.Iterable[yamcs.model.Client]
        """
        # Server does not do pagination on listings of this resource.
        # Return an iterator anyway for similarity with other API methods
        url = '/clients'
        if instance:
            url = '/instances/{}/clients'.format(instance)
        response = self.get_proto(path=url)
        message = rest_pb2.ListClientsResponse()
        message.ParseFromString(response.content)
        clients = getattr(message, 'client')
        return iter([Client(client) for client in clients])