print all post meta keys for current page site wide if in debug mode with query string.

debug-meta-keys-function.php

/**
 * Display all meta keys for the current post if the 'debug_meta' query string is set
 * and the site is in debug mode.
 *
 * This function is intended to be used on single post pages. It retrieves all meta keys for the
 * current post and displays them inside a `<div>` element if the 'debug_meta' parameter is present 
 * in the URL and WP_DEBUG is set to true. The meta keys are outputted in a readable format for debugging purposes.
 *
 * @return void
 */
function display_all_meta_keys_for_post() {
    // Check if we're on a single post page, WP_DEBUG is enabled, and the query string 'debug_meta' is present
    if (is_single() && defined('WP_DEBUG') && WP_DEBUG && isset($_GET['debug_meta'])) {
        global $post;
        $post_id = $post->ID;
        $all_meta = get_post_meta($post_id);

        if (!empty($all_meta)) {
            echo '<div class="post-meta-keys"><h3>Meta Keys and Values:</h3>';
            echo '<pre>';
            print_r($all_meta);
            echo '</pre>';
            echo '</div>';
        } else {
            echo '<div class="post-meta-keys"><p>No meta keys found for this post.</p></div>';
        }
    }
}
add_action('wp_footer', 'display_all_meta_keys_for_post');