Feature request - tail logs for google instance startup scripts

_This issue was originally opened by @JorritSalverda as hashicorp/packer#5820. It was migrated here as a result of the [Packer plugin split](https://github.com/hashicorp/packer/issues/8610#issuecomment-770034737). The original body of the issue is below._

With Google Compute Instances you can run startup scripts via metadata. This very useful to avoid issues caused by running installation steps over winrm in case of baking Windows images. However currently the output of those scripts is not tailed by Packer.

It is possible though to get the logs from all startup steps and executed scripts with
```
gcloud compute instances get-serial-port-output
```

It would be very useful to tail those in Packer so you can get feedback on failures in a startup script.