Add reading time to post content with the_content filter.

reading-time-by-the-content-filter.php

<?php

add_filter("the_content", function($content, $post){
  
  if ( is_singular() && in_the_loop() && is_main_query() ) {
    
    // https://gist.github.com/EdwardBock/fa659b67bcf4f568289d0c49c175f42b
    $add = reading_time_get_content($post->ID);
    
    return $add.$content; 
  }
  
  return $content;
});