Harden prometheus systemd service

prometheus service state BEFORE this change:

| % systemd-analyze security prometheus.service | tail -1
| → Overall exposure level for prometheus.service: 9.2 UNSAFE 😨

prometheus service state AFTER this change:

| % sudo systemd-analyze security prometheus.service | tail -1
| → Overall exposure level for prometheus.service: 3.8 OK 🙂

Closes: #950759