message: stats: fix stat flush crash during hot restart
  If histogram merge completion happens after admin shutdown, there is
  no longer any refresh timer and we crash. Guard the timer enable.
diff: @@ -156,7 +156,9 @@ void InstanceImpl::flushStats() {
  sslContextManager().daysUntilFirstCertExpires());
  InstanceUtil::flushMetricsToSinks(config_->statsSinks(), stats_store_);
  // TODO(ramaraochavali): consider adding different flush interval for histograms.
  + if (stat_flush_timer_ != nullptr) {
  stat_flush_timer_->enableTimer(config_->statsFlushInterval());
  + }
  });
  }
  @@ -459,6 +461,9 @@ void InstanceImpl::shutdown() {
  void InstanceImpl::shutdownAdmin() {
  ENVOY_LOG(warn, "shutting down admin due to child startup");
  + // TODO(mattklein123): Since histograms are not shared between processes, this will also stop
  + // histogram flushing. In the future we can consider whether we want to
  + // somehow keep flushing histograms from the old process.
  stat_flush_timer_.reset();
  handler_->stopListeners();
  admin_->mutable_socket().close();
  