apt update
# Stretch:
apt install -t stretch-backports prometheus
# Buster:
apt install prometheus

# On main prom1 server, running buster:
apt install prometheus-alertmanager prometheus prometheus-node-exporter

# Open up ports on firewall:
# Prometheus
-A INPUT -p tcp --dport 9090 -j ACCEPT
-A INPUT -p tcp --dport 9093 -j ACCEPT
-A INPUT -p tcp --dport 9094 -j ACCEPT
-A INPUT -p tcp --dport 9100 -j ACCEPT