Add experimental alert for 20 percent guild member growth

Author: jb3

kubernetes/namespaces/monitoring/alerts/alerts.d/discord.yaml

@@ -11,3 +11,16 @@ groups:
     annotations:
       summary: "High rate of guild member joins"
       description: "User join rate is {{ $value }} per hour, which may indicate a raid or other unusual activity."
+
+  # Experimental alert that checks for 20% growth based on previous period instead of static rate measurement
+  # Hopefully, this is more adaptable as server growth rates vary.
+  - alert: discord/experimental-members-growth
+    expr: |
+      delta(bot_guild_total_members[30m]) > 1.2 * delta(bot_guild_total_members[30m] offset 30m)
+      and delta(bot_guild_total_members[30m] offset 30m) > 0
+    for: 5m
+    labels:
+      severity: page
+    annotations:
+      summary: "Experimental: Unusual growth in guild members"
+      description: "Guild member count has increased by more than 20% compared to the previous 30-minute period, which may indicate a raid or other unusual activity."