update PushTraceSpans to return dropped spans instead of good spans. (#251)

* update PushTraceSpans to return dropped spans instead of good spans.

fixez #250

* return len(spans) if err is not nil

Author: rghetia

stackdriver.go

@@ -429,9 +429,10 @@ func (e *Exporter) ExportSpan(sd *trace.SpanData) {
 	e.traceExporter.ExportSpan(sd)
 }
 
-// PushTraceSpans exports a bundle of OpenCensus Spans
+// PushTraceSpans exports a bundle of OpenCensus Spans.
+// Returns number of dropped spans.
 func (e *Exporter) PushTraceSpans(ctx context.Context, node *commonpb.Node, rsc *resourcepb.Resource, spans []*trace.SpanData) (int, error) {
-	return len(spans), e.traceExporter.pushTraceSpans(ctx, node, rsc, spans)
+	return e.traceExporter.pushTraceSpans(ctx, node, rsc, spans)
 }
 
 func (e *Exporter) sdWithDefaultTraceAttributes(sd *trace.SpanData) *trace.SpanData {

trace.go

@@ -120,7 +120,7 @@ func (e *traceExporter) Flush() {
 	e.bundler.Flush()
 }
 
-func (e *traceExporter) pushTraceSpans(ctx context.Context, node *commonpb.Node, r *resourcepb.Resource, spans []*trace.SpanData) error {
+func (e *traceExporter) pushTraceSpans(ctx context.Context, node *commonpb.Node, r *resourcepb.Resource, spans []*trace.SpanData) (int, error) {
 	ctx, span := trace.StartSpan(
 		ctx,
 		"contrib.go.opencensus.io/exporter/stackdriver.PushTraceSpans",
@@ -148,7 +148,12 @@ func (e *traceExporter) pushTraceSpans(ctx context.Context, node *commonpb.Node,
 	ctx, cancel := newContextWithTimeout(ctx, e.o.Timeout)
 	defer cancel()
 
-	return e.client.BatchWriteSpans(ctx, &req)
+	err := e.client.BatchWriteSpans(ctx, &req)
+
+	if err != nil {
+		return len(spans), err
+	}
+	return 0, nil
 }
 
 // uploadSpans uploads a set of spans to Stackdriver.