Check that point value types match the descriptor type.

def _check_type(self):
        """Check that point value types match the descriptor type."""
        check_type = metric_descriptor.MetricDescriptorType.to_type_class(
            self.descriptor.type)
        for ts in self.time_series:
            if not ts.check_points_type(check_type):
                raise ValueError("Invalid point value type")