code: def get_metadata(doi):
      
      url = crossref_url + 'works/' + doi
      res = requests.get(url)
      if res.status_code != 200:
          logger.info('Could not get CrossRef metadata for DOI %s, code %d' %
                      (doi, res.status_code))
          return None
      raw_message = res.json()
      metadata = raw_message.get('message')
      return metadata
docs: Returns the metadata of an article given its DOI from CrossRef
      as a JSON dict