'Adds (a prefixed version of) the DatasetInstance metadata to the dict,
prefixing each key with \'metadata_\'.'

def _prefixed_metadata(self, dataset_assoc):

    metadata = self.serialize_metadata(dataset_assoc, 'metadata')
    prefixed = {}
    for (key, val) in metadata.items():
        prefixed_key = ('metadata_' + key)
        prefixed[prefixed_key] = val
    return prefixed
