It seems like terraform is not expecting expires_at to be null for service tokens, but when a service token is redeemed and activated, expires_at will turn to null as there will no longer be an expiration date on the token.
terraform import equinix_metal_connection.connect_foo {uuid}
equinix_metal_connection.connect_foo: Importing from ID "{uuid}"...
equinix_metal_connection.connect_foo Import prepared!
Prepared equinix_metal_connection for import
equinix_metal_connection.connect_foo: Refreshing state... [id={uuid}]
╷
│ Error: parsing time "null" as "2006-01-02T15:04:05Z07:00": cannot parse "null" as "2006"
│
Bug reported in terraform-provider-equinix. Datasource metal_connection here must be fixed as well.
https://github.com/equinix/terraform-provider-equinix/issues/147