Update custom field value

Something like this should do:

function track_editor_wpse_108606() {
  if (is_single()) {
    $current_user = wp_get_current_user();
    if (user_can( $current_user, 'editor' )) {
      global $post;
      $current = get_post_meta($post->ID,'editor_tracker',true);
      $current = (!empty($current)) ? $current + 1 : 1; 
      update_post_meta($post->ID,'editor_tracker',$current);
    }
  }
}
add_action('wp_head','track_editor_wpse_108606');

This is not especially efficient. You are reading and writing to the database for these page loads. You could improve this by means of a direct SQL query if you want to go that somewhat risky route, and could improve it further by using Javascript to insert the value if you don’t mind that it won’t work if the user disables Javascript.