How to update an instance metadata in GCP.

gcloud compute instances add-metadata instance-name \
--metadata key-name=value \
--project your-project-id \
--zone us-central1-a

In this example, we are adding enable-oslogin=TRUE.

gcloud compute instances add-metadata instance-name \
--metadata enable-oslogin=TRUE \
--project your-project-id \
--zone us-central1-a