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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 _initialize_metadata_service(cls):
 """Initialize metadata service once and load gcp metadata into map This method should only be called once. """

        if cls.inited:
            return

        instance_id = cls.get_attribute('instance/id')

        if instance_id is not None:
            cls.is_running = True

            _GCP_METADATA_MAP['instance_id'] = instance_id

            # fetch attributes from metadata request
            for attribute_key, attribute_uri in _GCE_ATTRIBUTES.items():
                if attribute_key not in _GCP_METADATA_MAP:
                    attribute_value = cls.get_attribute(attribute_uri)
                    if attribute_value is not None:  # pragma: NO COVER
                        _GCP_METADATA_MAP[attribute_key] = attribute_value

        cls.inited = True