functions.php

function start_wp_head_buffer() {
    ob_start();
}
add_action('wp_head','start_wp_head_buffer',0);

function end_wp_head_buffer() {
    $in = ob_get_clean();
    echo $in; // output to head-tag
}
add_action('wp_head','end_wp_head_buffer', PHP_INT_MAX);

add_action('rest_api_init', function () {
    register_rest_route( 'hs/v1', 'header',array(
    'methods'  => 'GET',
        'callback' => 'end_wp_head_buffer'
    ));
});