How to use identity.metadata with k8s auth?

We have a multi-tenant service in kubernetes. And we want to use the name of the tenant as part of the path to the secret. In addition, we want to have a templated policy.
I see that there is a metadata field in the identity. In the tutorials, various custom fields are put there.
We want to add a field to the identity metadata with the name of the tenant in order to look at it in the policy.
I don’t understand how to achieve this. Where can this field be described? Help me please.

Identity metadata is written using the API described at Identity Secret Backend: Entity - HTTP API | Vault by HashiCorp