Switch SD exporter to polling

[c24t]

This PR adds support for polling metrics from the stackdriver exporter. It builds on #560, which removed the ability to push metrics to the exporter via MeasureToViewMap.record.

I've added a PeriodicTask class (similar to the java client's IntervalMetricReader) that periodically calls exporter.export(producer.get_metrics()). To keep the API changes to a minimum I've changed new_stats_exporter to create and start the task, but we may want to change this to make the exporter ignorant of the transport instead.

@songy23 and @colincadams I'm interested to hear your thoughts on the API changes here.

[c24t]

Note that this transport serves a different purpose than opencensus.common.transports.

On master:

exporter.export ->  exporter.transport.export -> exporter.emit -> ... -> exporter.create_time_series

vs. on this branch:

background_thread -> exporter.export -> exporter.create_time_series

I'm open to a different name or structure here.

[songy23]

This serves a similar purpose to https://github.com/census-instrumentation/opencensus-go/blob/master/metric/metricexport/reader.go#L62, what about interval_metric_reader?

[colincadams]

High level question about this flow since I'm a little behind how stackdriver-metrics works. What happens when measure_to_view_map.export is called now? I see it is deprecated is it a no-op somehow?

[c24t]

Nothing! We should be able to remove this and many other supporting classes (e.g. ViewData) once stackdriver and prometheus are migrated.

[c24t]

@songy23 it's long, but maybe worth it for the consistency between packages.

[songy23]

Overall APIs look good, happy to see export in Python becomes more consistent with other languages.

[songy23]

This serves a similar purpose to https://github.com/census-instrumentation/opencensus-go/blob/master/metric/metricexport/reader.go#L62, what about interval_metric_reader?

[colincadams]

Overall I like this! Would the goal be to eventually do this for all exporters, once stackdriver-metrics is complete and merged in?

[c24t]

Would the goal be to eventually do this for all exporters

All stats/metrics exporters at least. The idea is that metrics are generated by producers, and the whole stats module is a single producer. The API for exporting traces will still let us push.