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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 _check_start_timestamp(self):
 """Check that starting timestamp exists for cumulative metrics."""

        if self.descriptor.type in (
                metric_descriptor.MetricDescriptorType.CUMULATIVE_INT64,
                metric_descriptor.MetricDescriptorType.CUMULATIVE_DOUBLE,
                metric_descriptor.MetricDescriptorType.CUMULATIVE_DISTRIBUTION,
        ):
            for ts in self.time_series:
                if ts.start_timestamp is None:
                    raise ValueError("time_series.start_timestamp must exist "
                                     "for cumulative metrics")