docs: def msg2usernames(msg, **config):
      ''' Return cached fedmsg.meta.msg2usernames(...) '''
  
      if not _cache.is_configured:
          _cache.configure(**config['fmn.rules.cache'])
  
      key = "|".join(['usernames', msg['msg_id']]).encode('utf-8')
      creator = lambda: fedmsg.meta.msg2usernames(msg, **config)
      return _cache.get_or_create(key, creator)