Test that MetricExporter.export() will not be called concurrently for the same instance

[aabmass]

As mentioned in the spec

Export will never be called concurrently for the same exporter instance. Export can be called again only after the current call returns.

~We already do this AFAIK, but want to add a test to ensure this behavior doesn't accidentally change~

After writing the test, it turns out we don't actually guarantee this. Putting up a PR soon