docs: @MustBeClosed
    public static Scope withScope(ServletRequest request) {
      checkNotNull(request, "request");
      OcHttpServletListener listener =
          (OcHttpServletListener) request.getAttribute(OPENCENSUS_SERVLET_LISTENER);
      if (listener != null) {
        return listener.withSpan();
      }
      return Tracing.getTracer().withSpan(BlankSpan.INSTANCE);
    }