Emit exports to the Prometheus if view data has one or more rows.
        Each OpenCensus AggregationData will be converted to
        corresponding Prometheus Metric: SumData will be converted
        to Untyped Metric, CountData will be a Counter Metric
        DistributionData will be a Histogram Metric.

def emit(self, view_data):  # pragma: NO COVER
        """ Emit exports to the Prometheus if view data has one or more rows.
        Each OpenCensus AggregationData will be converted to
        corresponding Prometheus Metric: SumData will be converted
        to Untyped Metric, CountData will be a Counter Metric
        DistributionData will be a Histogram Metric.
        """

        for v_data in view_data:
            if v_data.tag_value_aggregation_data_map:
                self.collector.add_view_data(v_data)