Extend Meta description

/**
 * Action: 'rank_math/vars/register_extra_replacements' - Allows adding extra variables.
* Snippet to register variable that will return all terms in custom taxonomy
 */
add_action( 'rank_math/vars/register_extra_replacements', function(){
        rank_math_register_var_replacement(
           'limit_description',
           [
           'name'        => esc_html__( 'name', 'rank-math' ),
           'description' => esc_html__( 'description', 'rank-math' ),
           'variable'    => 'limit_description',
           'example'     => limit_description_callback(),
           ],
           'limit_description_callback'
   );
});
function limit_description_callback(){
        $content = get_the_content();
        return substr(strip_tags($content), 0, 300);
}