Изменено имя аргумента сервера хранения, рабочий синтаксис:
[Unit]
Description=Prometheus Server
After=network-online.target
[Service]
User=root
Restart=on-failure
ExecStart=/usr/local/bin/prometheus-2.2.1.linux-amd64/prometheus \
--config.file=/etc/prometheus/prometheus.yml \
--storage.tsdb.path=/var/lib/prometheus/data
[Install]
WantedBy=multi-user.target