'Increments counter value by a given delta.'

def Increment(self, delta, fields=None):

    if (delta < 0):
        raise ValueError(('Delta    should    be    >    0    (not    %d)' % delta))
    key = self._FieldsToKey(fields)
    if (key in self._values):
        self._values[key] += delta
    else:
        self._values[key] = delta
