WordPress: An example for adding noindex attribute to a page

functions.php

<?php
add_action( 'wp_head', function() {
  if(!(is_single() || is_page())){
    return;
  }
  $value = get_post_meta(get_the_ID(), 'is_noindex', true );
  if(!((int)$value)) {
    return;
  }
  wp_no_robots();
});