Add value to key-value
        Params:
            <str> key
            <int> value
            <int> key_length
        Return:
            <int> key_value

def inc(self, key, key_length=0):
        """Add value to key-value
        Params:
            <str> key
            <int> value
            <int> key_length
        Return:
            <int> key_value
        """
        if key_length < 1:
            key_length = len(key)
        val = self.add_method(self, key, key_length, 1)
        if self.k:
            self._update(key, val)
        return val