modifying fleet metadata (from aws meta-data service) before fleet.service start; this is a proof of concept (but it works!)

cloud-config.yml

#cloud-config
---
coreos:
  units:
  - name: update-fleet-metadata.service
    command: start
    content: |-
      [Unit]
      Description=Update Fleet metadata tag
      Before=fleet.service
      Requires=fleet.service
      [Service]
      Type=oneshot
      RemainAfterExit=no
      EnvironmentFile=/etc/environment
      ExecStart=/bin/bash -c '\
          conf_file=/run/systemd/system/fleet.service.d/21-metadata.conf; \
          instance_id=$(curl -s http://169.254.169.254/latest/meta-data/instance-id); \
          az=$(curl -s http://169.254.169.254/latest/meta-data/placement/availability-zone); \
          region=$${az::-1}; \
          sgs=$(curl -s http://169.254.169.254/latest/meta-data/security-groups/ | sed "s/^/sg_/" | sed "s/$/=1/" | tr "\\n", ","); \
          echo "[Service]" > $conf_file; \
          echo "Environment=\'FLEET_METADATA=instance_id=$${instance_id},az=$${az},region=$${region},$${sgs}\'"  >> $conf_file; \
          systemctl daemon-reload; \
      '

I had to create the directory before I could write out the file... https://gist.github.com/brodkin/b4fbcdc172cf8ddd06cf
Once I did that it worked perfectly!

I added some tweaks for additional meta info: https://gist.github.com/cbsmith/e94e7525906bbe3b3be6