def tag_string(message):
    """Convert a gRPC V3Tag message to its corresponding tag string.

    Args:
        message (V3Tag): The tag message to convert to its string representation.

    Returns:
        str: The string representation of the tag message.
    """
    tag = ''
    if message.namespace:
        tag += f'{message.namespace}/'
    if message.annotation:
        tag += f'{message.annotation}:'
    tag += message.label
    return tag
