Provide the number of items in a queue

    CLI Example:

    .. code-block:: bash

        salt-run queue.list_length myqueue
        salt-run queue.list_length myqueue backend=sqlite

def list_length(queue, backend='sqlite'):
    '''
    Provide the number of items in a queue

    CLI Example:

    .. code-block:: bash

        salt-run queue.list_length myqueue
        salt-run queue.list_length myqueue backend=sqlite
    '''
    queue_funcs = salt.loader.queues(__opts__)
    cmd = '{0}.list_length'.format(backend)
    if cmd not in queue_funcs:
        raise SaltInvocationError('Function "{0}" is not available'.format(cmd))
    ret = queue_funcs[cmd](queue=queue)
    return ret