def metadata_filter_as_dict(metadata_config):
    """Return the metadata filter represented as either None (no filter),
    or a dictionary with at most two keys: 'additional' and 'excluded',
    which contain either a list of metadata names, or the string 'all'"""

    if metadata_config is None:
        return {}

    if metadata_config is True:
        return {'additional': 'all'}

    if metadata_config is False:
        return {'excluded': 'all'}

    if isinstance(metadata_config, dict):
        assert set(metadata_config) <= set(['additional', 'excluded'])
        return metadata_config

    metadata_keys = metadata_config.split(',')

    metadata_config = {}

    for key in metadata_keys:
        key = key.strip()
        if key.startswith('-'):
            metadata_config.setdefault('excluded', []).append(key[1:].strip())
        elif key.startswith('+'):
            metadata_config.setdefault('additional', []).append(key[1:].strip())
        else:
            metadata_config.setdefault('additional', []).append(key)

    for section in metadata_config:
        if 'all' in metadata_config[section]:
            metadata_config[section] = 'all'
        else:
            metadata_config[section] = [key for key in metadata_config[section] if key]

    return metadata_config