[Install]
WantedBy=multi-user.target

[Unit]
Description=Cloud SQL Proxy
Requires=networking.service
After=networking.service

[Service]
Type=simple
WorkingDirectory=/opt
ExecStart=/opt/cloud_sql_proxy -instances=pupa=tcp:5432 -credential_file=/etc/pupa-240309-5336639b0c06.json
Restart=always
StandardOutput=journal
User=root

Failed to start cloud-sql-proxy.service: Unit not found.

$ ll -al /etc/systemd/system/cloud-sql-proxy.service
-rw-r--r--. 1 root root 327 May 22 10:56 /etc/systemd/system/cloud-sql-proxy.service

$ sudo systemctl list-unit-files
...
cloud-sql-proxy.service                       enabled
...

$ systemctl status cloud-sql-proxy
● cloud-sql-proxy.service - Cloud SQL Proxy
   Loaded: loaded (/etc/systemd/system/cloud-sql-proxy.service; enabled; vendor preset: disabled)
   Active: failed (Result: resources) since Sun 2019-05-19 15:49:48 UTC; 3 days ago
 Main PID: 1414 (code=exited, status=1/FAILURE)

May 19 15:49:48 instance-1 systemd[1]: cloud-sql-proxy.service: main process exited, code=exited, status=1/FAILURE
May 19 15:49:48 instance-1 systemd[1]: Unit cloud-sql-proxy.service entered failed state.
May 19 15:49:48 instance-1 systemd[1]: cloud-sql-proxy.service failed.
May 19 15:49:48 instance-1 systemd[1]: cloud-sql-proxy.service holdoff time over, scheduling restart.
May 19 15:49:48 instance-1 systemd[1]: cloud-sql-proxy.service failed to schedule restart job: Unit not found.
May 19 15:49:48 instance-1 systemd[1]: Unit cloud-sql-proxy.service entered failed state.
May 19 15:49:48 instance-1 systemd[1]: cloud-sql-proxy.service failed.