'Records request for collecting data for the given xBlock fields.
A Transformer should call this method when it needs to collect
data for a common xBlock field that may also be used by other
transformers.  This minimizes storage usage across transformers.
Contrast this with each transformer collecting the same xBlock
data within its own transformer data storage.
Arguments:
field_names (list(string)) - A list of names of common
xBlock fields whose values should be collected.'

def request_xblock_fields(self, *field_names):

    self._requested_xblock_fields.update(set(field_names))
