How to use cloud-init with a debian-based image on Google Cloud?

Following terraform best practice for bootstrapping instances, I'm working on a cloud-init config in order to bootstrap my instance. My only need is to install a specific package.

My terraform config looks like this:

resource "google_compute_instance" "bastion" {
  name         = "my-first-instance"
  machine_type = "n1-standard-1"
  zone         = "europe-west1-b"

  boot_disk {
    initialize_params {
      image = "debian-cloud/debian-9"
    }
  }

  network_interface {
    network = "default"

    access_config {
      // Ephemeral IP
    }
  }

  metadata = {
    ssh-keys = "eugene:${file("/Users/eugene/.ssh/id_rsa.pub")}"
    user-data = file("my_cloud_init.conf")
  }
}

Following example for installing packages from cloud-init docs, here's the contents of my_cloud_init.conf:

#cloud-config

packages:
 - kubectl

After running terraform plan -out myplan and terraform apply myplan, I ssh onto the node only to find kubectl not available. Moreover, there's no evidence that cloud-init was run or that it exists on the node:

$ which -a cloud-init
$ cat /var/log/cloud-init
cat: /var/log/cloud-init: No such file or directory

Looking for clues about usage of cloud-init with Google Cloud Compute instances wasn't fruitful:

• "Google Cloud Engine" page from cloud-init docs suggests settings user-data to a cloud-init config should be enough,
• I see a cloud-init tutorial, but it's for Container Optimized OS,
• there are some clues about cloud-init on other images, but nothing indicates cloud-init is available on debian-cloud/debian-9,
• there's "Running startup scripts", but it has no mention of cloud-init.

I don't mind using another image, as long as it's Debian or Ubuntu and I don't have to make an image template myself.

How to use cloud-init with a debian-based image on Google Cloud? What am I missing?

cloud-init is installed on the latest (at the moment of writing) Ubuntu 18.04 LTS (ubuntu-1804-bionic-v20191002) image :

<my_user>@instance-1:~$ cat /etc/lsb-release
DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=18.04
DISTRIB_CODENAME=bionic
DISTRIB_DESCRIPTION="Ubuntu 18.04.3 LTS"

<my_user>@instance-1:~$ which cloud-init
/usr/bin/cloud-init

You should replace debian-cloud/debian-9 with ubuntu-os-cloud/ubuntu-1804-bionic-v20191002.