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