Add Shutdown() to Metric Exporter interface

[evantorrie]

What are you trying to achieve?

I propose adding a Shutdown() or Stop() method to the Metric Exporter SDK interface.

Currently, the Exporter for metrics in the SDK does not have a Shutdown method, whereas the trace SpanExporter does require it

Additional context.

We have an in-house push-based metrics exporter which queues checkpointed records to an asynchronous batched sender.

In the go implementation, the pusher implementation has a Stop() method, which sends one final checkpoint of records to the metric exporter. However, it has no way to notify the exporter that it is stopping - i.e. that the exporter should also at least flush/shut down itself as well.

It seems somewhat inconsistent that SpanExporters have a Shutdown() method but metrics Exporters do not.

[tedsuo]

Hi @evantorrie; FYI we're rebooting the metrics API WG. The current SDK spec has been removed from the spec, @reyang Is heading a working group that will add it back in once the prototypes are finished. Assigning this to him so we don't lose the requirement.

[reyang]

Fixed by #1864.