Hi @Shinra,
Thank you for your message. You're totally right, I said I'll write a blog note about custom metrics but I forgot, sorry for that. I definitely have to write it!
In the meantime, I can point you to the right direction: You have to use Meter node (https://backstage.forgerock.com/docs/auth-node-ref/latest/auth-node-meter.html) in your user journey to add counter metrics, or Timer Start node (https://backstage.forgerock.com/docs/auth-node-ref/latest/auth-node-timer-start.html) and Timer Stop node (https://backstage.forgerock.com/docs/auth-node-ref/latest/auth-node-timer-stop.html) to mesure the time elapsed between theses 2 nodes.
I hope it'll help you to start and I'll do my best to write a note about it shortly,
best regards,