Add "recipient" to subscriptions export, for use with Gifting for WooCommerce Subscriptions

recipient_custom_field.php

<?php
/**
 * Add custom headers to the list of default headers exported in the CSV
 *
 * @param array $headers
 * @return array
 */
function my_custom_export_headers( $headers = array() ) {
    return array_merge( $headers, array(
        '_recipient_user' => 'Recipient',
    ) );
}
add_filter( 'wcsie_export_headers', 'my_custom_export_headers', 10, 1 );

/**
 * Adds a custom meta value to the exported row
 *
 * @param string value
 * @param WC_Subscription $subscription
 * @param
 * @return string
 */
 function my_custom_export_values( $value, $subscription, $header_key ) {
   error_log( print_r( $subscription, true ) );
    if ( '_recipient_user' == $header_key && empty( $value ) ) {
        $value = WCS_Gifting::get_recipient_user( $subscription );
    }

    return $value;
 }
add_filter( 'wcsie_format_export_value', 'my_custom_export_values', 10, 3 );

If you are trying to use the Subscriptions exporter, and see which subscriptions have been gifted, you will need this snippet.
Here's the exporter: https://github.com/woocommerce/woocommerce-subscriptions-importer-exporter#subscriptions-exporter

This snippet will add a "Recipient" column, which will show the recipient user ID for any gifted subs.