Use MetricServiceServer in fakeMetricService. (#168)
Use MetricServiceServer in fakeMetricService. (#168)

* Use MetricServiceServer in fakeMetricService.

Fixes #167.

* Remove uncalled fake methods and type assertion.

Given that the fakeMetricService includes the MetricServiceServer interface, it necessarily implements the interface, even if some of the methods are nil and would panic if called. Uncalled methods are removed.

census-ecosystem/opencensus-go-exporter-stackdriver,census-ecosystem/opencensus-go-exporter-stackdriver