fix: dont send metadata on subscribe (#1648)

* fix: dont send metadata on subscribe

* fix: avoid warnings

* docs: add more inline docs

includes/class-newspack-newsletters-contacts.php

@@ -52,7 +52,17 @@ public static function subscribe( $contact, $lists = false, $async = false, $con
 		$existing_contact = Newspack_Newsletters_Subscription::get_contact_data( $contact['email'], true );
 		$is_updating      = \is_wp_error( $existing_contact ) ? false : true;
 
-		$result = self::upsert( $contact, $lists, $context, $existing_contact );
+		// When subscribing, we only want to keep the status and name metadata.
+		// Additional metadata can only be added when upserting a contact.
+		// This method is specific for handling Newsletter subscription, in which case there is no additional metadata being passed.
+		// Any additional metadata will be passes to the logs and filters though, so other actions can act upon it.
+		$accepted_metadata = [ 'status', 'name' ];
+		$subscribe_contact = $contact;
+		if ( ! empty( $subscribe_contact['metadata'] ) ) {
+			$subscribe_contact['metadata'] = array_intersect_key( $subscribe_contact['metadata'], array_flip( $accepted_metadata ) );
+		}
+
+		$result = self::upsert( $subscribe_contact, $lists, $context, $existing_contact );
 
 		if ( is_wp_error( $result ) ) {
 			return $result;