Send checkout page custom field in order email

Finally i got the answer i was wrong because i tried to send the variable you can not send the variable you have to just put the name of custom field slug.

In the bellow snippet i have replaced $field with my custom field name slug _some_field

like:- $keys['How did you learn about us?'] = '_some_field';

/**
 * Add the field to order emails
 **/
add_filter('woocommerce_email_order_meta_keys', 'my_woocommerce_email_order_meta_keys');

function my_woocommerce_email_order_meta_keys( $keys ) {
    $keys['How did you learn about us?'] = '_some_field';
    return $keys;
}