Create xfs partitions on Hetzner via cloud-init. It keeps root disk available again after rebooting. Simply change your desired sizes and filesystem to use it for your needs.

xfs-on-hetzner.yml

#cloud-config
resize_rootfs: false

disk_setup:
  /dev/sda:
    table_type: 'mbr'
    layout:
      - 25
      - 75
    overwrite: true

fs_setup:
  - label: root_fs
    filesystem: 'ext4'
    device: /dev/sda
    partition: sda1
    overwrite: true
  - label: data_disk
    filesystem: 'xfs'
    device: /dev/sda
    partition: sda2
    overwrite: true

runcmd:
  - [ partx, --update, /dev/sda ]
  - [ mkfs.xfs, /dev/sda2 ]
  - [ partprobe ]
  - parted /dev/sda set 1 boot on p

mounts:
  - ["/dev/sda1", "/"]
  - ["/dev/sda2", "/data_disk"]

The @ilyangru example very nearly worked for me as is with an Ubuntu 20.04 box, I just had to quote the "1" in the 5th runcmd. It took me longer than I'd like to figure out so hopefully that'll help someone else.

#cloud-config
# Ubuntu 18.04+
resize_rootfs: false
write_files:
  - content: |
    # Any text
    path: /etc/growroot-disabled

runcmd:
  - [ sgdisk, -e, /dev/sda ]
  - [ partprobe ]
  - [ parted, -s, /dev/sda, mkpart, primary, xfs, "25%", "100%" ]
  - [ mkfs.xfs, /dev/sda2 ]
  - [ growpart, /dev/sda, "1" ] 
  - [ resize2fs, /dev/sda1 ]

mounts:
 - [ /dev/sda2, /var/lib/mongodb ]