def METHOD_NAME(source_dir):
    version_metadata = os.path.join(source_dir, "version_metadata.json")
    with open(version_metadata, 'r') as file:
        data = file.read()
    obj = json.loads(data)
    version = obj.get('version_number')
    if version is None:
        log_message(logging.WARN, "Version number is not found in version_metadata.json.")
        return get_default_release_version()
    build_num = obj.get('build_number')
    if build_num is None:
        log_message(logging.WARN, "Build number is not found in version_metadata.json.")
        return get_default_release_version()
    version_format = "{}-b{}" if build_num.isdigit() else "{}-{}"
    return version_format.format(version, build_num)