Per service annotation - upstream client certificate

Hi,

I do like to use a certificate so i can call my upstream external service.
Im reading this documentation : Using mTLS with Kong - v1.3.x | Kong Docs.

It is mentioned :
(To use the annotation, you first need to create a TLS secret with the client certificate and key in Kubernetes. The secret should be created in the same namespace as your Kubernetes Service to which Kong should authenticate itself).

Its seems that we can use this annotation : konghq.com/client-cert:

I do like to get a good example with steps.

Thanks.