Get a running task that periodically exports metrics.

    Get a `PeriodicTask` that periodically calls:

        exporter.export_metrics(metric_producer.get_metrics())

    :type metric_producer:
        :class:`opencensus.metrics.export.metric_producer.MetricProducer`
    :param exporter: The producer to use to get metrics to export.

    :type exporter: :class:`opencensus.stats.base_exporter.MetricsExporter`
    :param exporter: The exporter to use to export metrics.

    :type interval: int or float
    :param interval: Seconds between export calls.

    :rtype: :class:`PeriodicTask`
    :return: A running thread responsible calling the exporter.

def get_exporter_thread(metric_producer, exporter, interval=None):
    """Get a running task that periodically exports metrics.

    Get a `PeriodicTask` that periodically calls:

        exporter.export_metrics(metric_producer.get_metrics())

    :type metric_producer:
        :class:`opencensus.metrics.export.metric_producer.MetricProducer`
    :param exporter: The producer to use to get metrics to export.

    :type exporter: :class:`opencensus.stats.base_exporter.MetricsExporter`
    :param exporter: The exporter to use to export metrics.

    :type interval: int or float
    :param interval: Seconds between export calls.

    :rtype: :class:`PeriodicTask`
    :return: A running thread responsible calling the exporter.

    """
    weak_get = utils.get_weakref(metric_producer.get_metrics)
    weak_export = utils.get_weakref(exporter.export_metrics)

    def export_all():
        get = weak_get()
        if get is None:
            raise TransportError("Metric producer is not available")
        export = weak_export()
        if export is None:
            raise TransportError("Metric exporter is not available")
        export(get())

    tt = MetricExporterTask(interval, export_all)
    tt.start()
    return tt