register-meta-new-1.php

<?php
register_meta('user', 'xp',
    [
        'sanitize_callback' => 'absint',
    ]
);

register-meta-new-2.php

<?php
register_meta('user', 'xp',
    [
        'sanitize_callback' => 'absint',
        'show_in_rest' => true,
    ]
);

register-meta-new-3.php

<?php
register_meta('user', 'xp',
    [
        'sanitize_callback' => 'absint',
        'show_in_rest' => true,
        'type' => 'integer',
        'description' => 'The total experience points earned by this user, as a positive integer',
    ]
);

register-meta-new-4.php

<?php
register_meta('user', 'xp',
    [
        'sanitize_callback' => 'absint',
        'show_in_rest' => true,
        'type' => 'integer',
        'description' => 'The total experience points earned by this user, as a positive integer',
        'auth_callback' => function(){
            return current_user_can( 'some-cap' );
        }
    ]
);

register-meta-old.php

<?php
register_meta( 'user', 'xp', 'absint' );