Hook register_meta() into WordPress 'init'

functions.php

<?php //* do not include php tag

add_action( 'init', 'register_my_sample_meta' );
/**
 * Callback for WordPress 'init' action.
 *
 * Register the post meta.
 *
 * @uses register_meta()
 * @see https://codex.wordpress.org/Function_Reference/register_meta
 * 
 * @return void
 */
function register_my_sample_meta() {
    
    register_meta(
        'post', //* the post type
        'sample_count', //* the meta key
        'sanitize_sample_count_meta', //* the callable sanitize function
        'auth_sample_count_meta' //* the optional authenticate function
    );
    
}