Add an alert for suspected DDoS

roles/ai3-prometheus/templates/rules/alerts_ddos.yml

+groups:
+- name: roles/ai3-prometheus/templates/rules/alerts_ddos.conf
+  rules:
+
+  - alert: DDoS
+    expr: sum((sum(instance:node_network_packets_total:rate5m{device=~"en.*"}) by (host)) * on (host) role{role="frontend"}) > 100000
+    for: 5m
+    labels:
+      scope: global
+      service: nginx
+      severity: page
+    annotations:
+      summary: 'Suspected incoming DDoS'
+      description: 'High network packet count on public frontends'
+      runbook: '[[ alert_runbook_fmt | format("DDoS") ]]'
+