<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 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