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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def sign(mv):
 """Obtains a signature for a `MetricValue` Args: mv (:class:`endpoints_management.gen.servicecontrol_v1_messages.MetricValue`):
 a MetricValue that's part of an operation Returns: string: a unique signature for that operation """

    md5 = hashlib.md5()
    update_hash(md5, mv)
    return md5.digest()