Update example to reflect the change from ExportView to ExportMetrics.

[songy23]

Prometheus exporter has been updated to use metric.reader to read and export Metric instead of view.Data. The old interface ExportView is already a no-op:

opencensus-go-exporter-prometheus/prometheus.go

Lines 108 to 110 in 71649c8

	// Deprecated in lieu of metricexport.Reader interface.
	func (e *Exporter) ExportView(vd *view.Data) {
	}

However in the example we still call the deprecated (and no-op) interface view.RegisterExporter and view.SetReportingPeriod

opencensus-go-exporter-prometheus/example/main.go

Line 46 in 71649c8

view.RegisterExporter(exporter)

opencensus-go-exporter-prometheus/example/main.go

Line 70 in 71649c8

view.SetReportingPeriod(1 * time.Second)

Example needs to be updated to reflect the change in metrics export.

/cc @yanweiguo