Stackdriver pricing for OSS Istio

Hello all,

According to https://cloud.google.com/stackdriver/pricing#monitoring-costs Istio metrics are not charged by Google.

My question is whether it is valid also for Open Source Istio (Stackdriver adapter enabled) or only for Google provided Istio add-on.

If it is only free for add-on, how Stackdriver distinguishes metrics coming from OSS or add-on?

Thanks.