Add NeverSample span around Stackdriver stats upload (#580)

Fixes: #573

exporter/stackdriver/stats.go

@@ -31,9 +31,10 @@ import (
 	"go.opencensus.io/internal"
 	"go.opencensus.io/stats/view"
 	"go.opencensus.io/tag"
+	"go.opencensus.io/trace"
 
-	monitoring "cloud.google.com/go/monitoring/apiv3"
-	timestamp "github.com/golang/protobuf/ptypes/timestamp"
+	"cloud.google.com/go/monitoring/apiv3"
+	"github.com/golang/protobuf/ptypes/timestamp"
 	"google.golang.org/api/option"
 	"google.golang.org/api/support/bundler"
 	distributionpb "google.golang.org/genproto/googleapis/api/distribution"
@@ -143,7 +144,7 @@ func getTaskValue() string {
 // handleUpload handles uploading a slice
 // of Data, as well as error handling.
 func (e *statsExporter) handleUpload(vds ...*view.Data) {
-	if err := e.upload(vds); err != nil {
+	if err := e.uploadStats(vds); err != nil {
 		e.onError(err)
 	}
 }
@@ -164,16 +165,24 @@ func (e *statsExporter) onError(err error) {
 	log.Printf("Failed to export to Stackdriver Monitoring: %v", err)
 }
 
-func (e *statsExporter) upload(vds []*view.Data) error {
-	ctx := context.Background()
+func (e *statsExporter) uploadStats(vds []*view.Data) error {
+	span := trace.NewSpan(
+		"go.opencensus.io/exporter/stackdriver.uploadStats",
+		nil,
+		trace.StartOptions{Sampler: trace.NeverSample()},
+	)
+	ctx := trace.WithSpan(context.Background(), span)
+	defer span.End()
 
 	for _, vd := range vds {
 		if err := e.createMeasure(ctx, vd); err != nil {
+			span.SetStatus(trace.Status{Code: 2, Message: err.Error()})
 			return err
 		}
 	}
 	for _, req := range e.makeReq(vds, maxTimeSeriesPerUpload) {
 		if err := e.c.CreateTimeSeries(ctx, req); err != nil {
+			span.SetStatus(trace.Status{Code: 2, Message: err.Error()})
 			// TODO(jbd): Don't fail fast here, batch errors?
 			return err
 		}