code: public function onOutput()
      {
          if ($this->debug->parentInstance) {
              // only record runtime info for root instance
              return;
          }
          $vals = $this->runtimeVals();
          $this->debug->groupSummary(1);
          $this->debug->info('Built In '.$vals['runtime'].' sec');
          $this->debug->info(
              'Peak Memory Usage'
                  .($this->debug->getCfg('output/outputAs') == 'html'
                      ? ' <span title="Includes debug overhead">?&#x20dd;</span>'
                      : '')
                  .': '
                  .$this->debug->utilities->getBytes($vals['memoryPeakUsage']).' / '
                  .$this->debug->utilities->getBytes($vals['memoryLimit'])
          );
          $this->debug->groupEnd();
      }
docs: debug.output event subscriber
  
  @return void