Add Flush to force aggregate in-memory buffer

[rakyll]

Fixes #862.

[odeke-em]

@rakyll thanks for also working on this! I had mailed #922 sometime back. However, be ware that calling Flush after any future stops shouldn't block :)

[bogdandrutu]

@rakyll I think (RetrieveData)[https://github.com/census-instrumentation/opencensus-go/blob/master/stats/view/worker.go#L94] is your answer for this. See how I used it https://github.com/census-instrumentation/opencensus-service/blob/master/observability/observabilitytest/observabilitytest.go#L105

Do you think that is enough?

[wdhnl]

Can this be looked at again?

[rakyll]

Closing this due to inactivity, please open another PR if it needs to be addressed.