writes user defined metadata for the given symbol

        Parameters
        ----------
        symbol: str
            symbol for the given item in the DB
        metadata: ?
            metadata to write

def write_metadata(self, symbol, metadata):
        '''
        writes user defined metadata for the given symbol

        Parameters
        ----------
        symbol: str
            symbol for the given item in the DB
        metadata: ?
            metadata to write
        '''
        sym = self._get_symbol_info(symbol)
        if not sym:
            raise NoDataFoundException("Symbol does not exist.")

        sym[USERMETA] = metadata
        self._symbols.replace_one({SYMBOL: symbol}, sym)