Don't report traces from stackdriver stats exporter

[groob]

In #557 it was reported that calls to v2.TraceService.BatchWriteSpans were being sent to GCP trace by default, creating a lot of noise. #560 fixed this issue for traces, but I still see it for stats.

Sent.google.monitoring.v3.MetricService.CreateTimeSeries

My code looks like this:

        exporter, err := stackdriver.NewExporter(stackdriver.Options{
                ProjectID:            *flProject,
                BundleDelayThreshold: 10 * time.Second,
        })
        if err != nil {
                logutil.Fatal(logger, "err", err)
        }
        trace.RegisterExporter(exporter)
        trace.SetDefaultSampler(trace.AlwaysSample())

        view.RegisterExporter(exporter)
        view.Subscribe(ochttp.DefaultViews...)
        view.SetReportingPeriod(1 * time.Second)

[bogdandrutu]

@Ramonza you should do the same thing as you did for trace.