Source

File: wp-includes/class-wp-xmlrpc-server.php

public function get_custom_fields( $post_id ) {
		$post_id = (int) $post_id;

		$custom_fields = array();

		foreach ( (array) has_meta( $post_id ) as $meta ) {
			// Don't expose protected fields.
			if ( ! current_user_can( 'edit_post_meta', $post_id, $meta['meta_key'] ) ) {
				continue;
			}

			$custom_fields[] = array(
				'id'    => $meta['meta_id'],
				'key'   => $meta['meta_key'],
				'value' => $meta['meta_value'],
			);
		}

		return $custom_fields;
	}