• Library version: latest (Currently 8.0)
• GKE 1.12

Steps to reproduce:

Use of the admin api within GKE 1.12 failed with error:
Your client does not have permission to get URL /computeMetadata/v1beta1/instance/service-accounts/default/token from this server. Legacy metadata endpoint accessed: /computeMetadata/v1beta1/instance/service-accounts/default/token

As noted here the legacy endpoints as now disabled by default:
https://cloud.google.com/kubernetes-engine/docs/how-to/protecting-cluster-metadata#disable-legacy-apis

Relevant Code:

I think this code should be updated to use/v1:

https://github.com/firebase/firebase-admin-node/blob/master/src/auth/credential.ts#L32