create labels for the metric_descriptor
        that will be sent to Stackdriver Monitoring

def new_label_descriptors(defaults, keys):
    """ create labels for the metric_descriptor
        that will be sent to Stackdriver Monitoring
    """
    label_descriptors = []
    for lk in itertools.chain.from_iterable((defaults.keys(), keys)):
        label = {}
        label["key"] = sanitize_label(lk.key)
        label["description"] = lk.description
        label_descriptors.append(label)

    return label_descriptors