Stats: flush data to exporters

[knyar]

The spec does not describe whether the stats library maintains an internal buffer before passing stats to exporters, but in practice Go and C++ libraries seem to do that (and other languages like Python, Ruby, PHP and Node don't have stats implemented yet at all).

As the result, for push-based exporters (like Stackdriver) there might be two layers of buffering: one in the stats library, the other in the exporter. Such exporters should probably allow explicit flushing of their buffers, but it will be valuable to provide users a way to flush the buffer of the stats library as well.

This can be done to avoid data loss when a given view is unregistered or the process shuts down. It will also probably make testing easier; currently tests need to sleep (1, 2).

Go users seem to work around this by decreasing reporting period (census-instrumentation/opencensus-go#773). C++ library does not allow changing the reporting period, which is hard-coded to be 10.

In census-instrumentation/opencensus-go#846 (where this discussion started) I initially proposed an explicit ReportUsage() function. An alternative (that is probably more user-friendly and does not expose as much implementation) is to require the stats libraries to provide functions to unregister/remove views and flush pending data to exporters when that happens.

[bogdandrutu]

I think this is related to the discussion about record semantics #159. Also I asked for a flush/shutdown function.

[semistrict]

Updated that PR with a description of Flush.