itamae/cookbooks/prometheus/files/etc/systemd/system/alertmanager.service

[Unit]
Description=Alertmanager
Wants=network-online.target
After=network-online.target

[Service]
Type=simple
ExecStart=/usr/local/bin/alertmanager --config.file /etc/prometheus.d/alertmanager.yml

[Install]
WantedBy=multi-user.target