internal/dcensus: downgrade stackdriver API errors to log warnings

Change-Id: I0e1e9e8bfb6e0d1bca59a0d598ee3e6efa096c1f
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/261820
Trust: Jonathan Amsterdam <jba@google.com>
Run-TryBot: Jonathan Amsterdam <jba@google.com>
Reviewed-by: Julie Qiu <julie@golang.org>
TryBot-Result: kokoro <noreply+kokoro@google.com>

internal/dcensus/dcensus.go

@@ -178,7 +178,7 @@ func NewViewExporter(cfg *config.Config) (_ *stackdriver.Exporter, err error) {
 		MonitoredResource:       mr,
 		DefaultMonitoringLabels: stackdriverLabels(cfg),
 		OnError: func(err error) {
-			log.Errorf(context.Background(), "Stackdriver view exporter: %v", err)
+			log.Warningf(context.Background(), "Stackdriver view exporter: %v", err)
 		},
 	})
 }

internal/dcensus/debug.go

@@ -35,7 +35,7 @@ func (d *debugTraceExporter) ExportSpan(s *trace.SpanData) {
 	d.err = nil
 	d.mu.Unlock()
 	if err != nil {
-		log.Errorf(ctx, "trace exporter: %v", err)
+		log.Warningf(ctx, "trace exporter: %v", err)
 		log.Debugf(ctx, "trace exporter SpanData:\n%s", dumpSpanData(s))
 	}
 }