code: void updateImageMetrics() {
      if (metrics == null) {
        return;
      }
      int failedImageDirs = 0;
      for (ImageManager im : imageManagers) {
        if(im.isImageDisabled()) {
          failedImageDirs++;
        }
      }
      // update only images, journals are handled in JournalSet
      metrics.imagesFailed.set(failedImageDirs);
    }
docs: Count available image managers and update namenode metrics.