Use StubInterface instead of Stub, for mocks. (#366)

e.g. MockMetricServiceStub and MetricService::Stub both inherit from
MetricService::StubInterface.

This change doesn't break the API.

Author: g-easy

opencensus/exporters/stats/stackdriver/stackdriver_exporter.h

@@ -77,7 +77,7 @@ struct StackdriverOptions {
   // Optional: by default, the exporter connects to Stackdriver using gRPC. If
   // this stub is non-null, the exporter will use this stub to send gRPC calls
   // instead. Useful for testing.
-  std::unique_ptr<google::monitoring::v3::MetricService::Stub>
+  std::unique_ptr<google::monitoring::v3::MetricService::StubInterface>
       metric_service_stub;
 };
 

opencensus/exporters/trace/stackdriver/stackdriver_exporter.h

@@ -37,7 +37,7 @@ struct StackdriverOptions {
   // (optional) By default, the exporter connects to Stackdriver using gRPC. If
   // this stub is non-null, the exporter will use this stub to send gRPC calls
   // instead. Useful for testing.
-  std::unique_ptr<google::devtools::cloudtrace::v2::TraceService::Stub>
+  std::unique_ptr<google::devtools::cloudtrace::v2::TraceService::StubInterface>
       trace_service_stub;
 };