Update gRPC measures and views from spec (#689)

* Update gRPC measures and views from spec
* Increase wait time after AppVeyor suprious failure
TestCumulativenessFromHistograms failed on AppVeyor. Looks likely
to be timing related. Without rewriting the test to avoid timing
issues, I am just increasing the wait times in the hopes that this
will resolve the issue.
* Replace stats.UnitNone with stats.UnitDimensionless.
* Add benchmark to test if map-based statusCodeToString is better

Fixes: #687

Author: Ramon Nogueira

exporter/stackdriver/stats.go

@@ -243,7 +243,7 @@ func (e *statsExporter) createMeasure(ctx context.Context, vd *view.Data) error
 		valueType = metricpb.MetricDescriptor_INT64
 		// If the aggregation type is count, which counts the number of recorded measurements, the unit must be "1",
 		// because this view does not apply to the recorded values.
-		unit = stats.UnitNone
+		unit = stats.UnitDimensionless
 	case view.AggTypeSum:
 		switch m.(type) {
 		case *stats.Int64Measure:

exporter/stackdriver/stats_test.go

@@ -685,13 +685,13 @@ func TestExporter_createMeasure_CountAggregation(t *testing.T) {
 		if got, want := mdr.MetricDescriptor.DisplayName, "OpenCensus/test_view_count"; got != want {
 			t.Errorf("MetricDescriptor.DisplayName = %q; want %q", got, want)
 		}
-		if got, want := mdr.MetricDescriptor.Unit, stats.UnitNone; got != want {
+		if got, want := mdr.MetricDescriptor.Unit, stats.UnitDimensionless; got != want {
 			t.Errorf("MetricDescriptor.Unit = %q; want %q", got, want)
 		}
 		return &metric.MetricDescriptor{
 			DisplayName: "OpenCensus/test_view_sum",
 			Description: "view_description",
-			Unit:        stats.UnitNone,
+			Unit:        stats.UnitDimensionless,
 			Type:        "custom.googleapis.com/opencensus/test_view_count",
 			MetricKind:  metricpb.MetricDescriptor_CUMULATIVE,
 			ValueType:   metricpb.MetricDescriptor_INT64,