Fix equalMeasureAggTagKeys for Stackdriver exporter. (#685)

Fixes census-instrumentation/opencensus-go#683.

Count aggregation will always be exported as INT64 MetricDescriptor, no
matter whether Measure is INT64 or FLOAT64.

Author: songy23

exporter/stackdriver/stats.go

@@ -404,7 +404,7 @@ func equalMeasureAggTagKeys(md *metricpb.MetricDescriptor, m stats.Measure, agg
 	var aggTypeMatch bool
 	switch md.ValueType {
 	case metricpb.MetricDescriptor_INT64:
-		if _, ok := m.(*stats.Int64Measure); !ok {
+		if _, ok := m.(*stats.Int64Measure); !(ok || agg.Type == view.AggTypeCount) {
 			return fmt.Errorf("stackdriver metric descriptor was not created as int64")
 		}
 		aggTypeMatch = agg.Type == view.AggTypeCount || agg.Type == view.AggTypeSum || agg.Type == view.AggTypeLastValue

exporter/stackdriver/stats_test.go

@@ -424,7 +424,7 @@ func TestEqualAggWindowTagKeys(t *testing.T) {
 		wantErr bool
 	}{
 		{
-			name: "count agg",
+			name: "count agg with in64 measure",
 			md: &metricpb.MetricDescriptor{
 				MetricKind: metricpb.MetricDescriptor_CUMULATIVE,
 				ValueType:  metricpb.MetricDescriptor_INT64,
@@ -434,6 +434,17 @@ func TestEqualAggWindowTagKeys(t *testing.T) {
 			agg:     view.Count(),
 			wantErr: false,
 		},
+		{
+			name: "count agg with double measure",
+			md: &metricpb.MetricDescriptor{
+				MetricKind: metricpb.MetricDescriptor_CUMULATIVE,
+				ValueType:  metricpb.MetricDescriptor_INT64,
+				Labels:     []*label.LabelDescriptor{{Key: opencensusTaskKey}},
+			},
+			m:       stats.Float64("name", "", ""),
+			agg:     view.Count(),
+			wantErr: false,
+		},
 		{
 			name: "sum agg double",
 			md: &metricpb.MetricDescriptor{