code: static void push(Span span, boolean autoClose) {
      if (isCurrent(span)) {
        return;
      }
      setSpanContextInternal(new SpanContext(span, autoClose));
    }
docs: Push a span into the thread context, with the option to have it auto close if any child spans
  are themselves closed. Use autoClose=true if you start a new span with a parent that wasn't
  already in thread context.