SRE-86: Add FreeableMemory CloudWatch alert for RDS

[TimDiekmann]

🌟 What is the purpose of this PR?

Add a CloudWatch alarm to monitor RDS freeable memory and alert when it drops below a critical threshold.

🔍 What does this change?

• Adds a new CloudWatch metric alarm rds_freeable_memory_low that triggers when the database instance has less than 1GB of freeable memory for 10 minutes
• Configures the alarm to send notifications to the existing database alerts SNS topic
• Sets appropriate tags for severity and purpose tracking

🛡 What tests cover this?

• Existing Terraform validation tests

❓ How to test this?

1. Apply the Terraform changes
2. Verify the new CloudWatch alarm appears in the AWS console
3. Confirm the alarm is properly linked to the SNS topic for notifications

Pre-Merge Checklist 🚀

🚢 Has this modified a publishable library?

This PR:

• does not modify any publishable blocks or libraries, or modifications do not need publishing

📜 Does this require a change to the docs?

The changes in this PR:

• are internal and do not require a docs change

🕸️ Does this require a change to the Turbo Graph?

The changes in this PR:

• do not affect the execution graph

[TimDiekmann]

• SRE-89: Update Cloudflare provider to version 5.12 #8010
• SRE-88: Enable AWS GuardDuty with PagerDuty notifications #8009
• SRE-67: Add AWS GuardDuty configuration for security monitoring #8006
• SRE-87: Add CloudWatch alarms for RDS I/O monitoring #8005
• SRE-86: Add FreeableMemory CloudWatch alert for RDS #8004 👈 (View in Graphite)
• SRE-80: Add CPUUtilization CloudWatch alert for RDS #8003
• SRE-79: Add FreeStorageSpace CloudWatch alert for RDS #8002
• SRE-82: Replace Slack alerts with PagerDuty integration for Grafana monitoring #8001
• main

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[graphite-app]

Graphite Automations

"Request DevOps reviewers once CI passes" took an action on this PR • (11/03/25)

1 reviewer was added to this PR based on Tim Diekmann's automation.

[graphite-app]

Merge activity

• Nov 4, 11:07 AM UTC: Graphite rebased this pull request, because this pull request is set to merge when ready.
• Nov 4, 11:07 AM UTC: Graphite rebased this pull request, because this pull request is set to merge when ready.