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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def Increment(self, delta, fields=None):
 """Increments counter value by a given delta."""

    if delta < 0:
      raise ValueError(
          "Counter increment should not be < 0 (received: %d)" % delta)

    self._metric_values[_FieldsToKey(fields)] = self.Get(fields=fields) + delta