Issue #470 e2e test prometheus exporter (#488)
Issue #470 e2e test prometheus exporter (#488)

* testing for known key/label duplication error and any other registry error

* loop until response body isn't empty

* creator consistency

* fatal if there is no output after 10 cycles

* report every ms and sleep for one ms per loop

* Test that expected measures are in /metrics

* Remove unused handler registration

* Break loop when there is output

* Stop checking twice for empty output

census-instrumentation/opencensus-go,census-instrumentation/opencensus-go,census-instrumentation/opencensus-go