code: public static void unregister() {
      synchronized (monitor) {
        checkState(handler != null, "Zipkin exporter is not registered.");
        unregister(Tracing.getExportComponent().getSpanExporter());
        handler = null;
      }
    }
docs: Unregisters the Zipkin Trace exporter from the OpenCensus library.
  
  @throws IllegalStateException if a Zipkin exporter is not registered.
  @since 0.12