docs: def read(self, symbol, as_of=None):
          """
          Return current metadata saved for `symbol`
  
          Parameters
          ----------
          symbol : `str`
              symbol name for the item
          as_of : `datetime.datetime`
              return entry valid at given time
  
          Returns
          -------
          metadata
          """
          if as_of is not None:
              res = self.find_one({'symbol': symbol, 'start_time': {'$lte': as_of}},
                                  sort=[('start_time', pymongo.DESCENDING)])
          else:
              res = self.find_one({'symbol': symbol}, sort=[('start_time', pymongo.DESCENDING)])
          return res['metadata'] if res is not None else None