Quick Question,
Most probably know the answer, but if using Multi-AZ RDS with MySql and the primary storage is full, then the standby replica will also be full as well? Failover would only be successful in this situation should the primary storage just have an issue, but not necessarily be full.
What alternatives would you use in this case besides Cloudwatch and RDS metric for storage?
Yes, you are correct regarding the standby storage space.
You should have your CloudWatch alerts configured to notify you well ahead of time. For example you could configure it to start sending you emails when the storage is at 80% full, and make plans to increase your storage space well before it hits 100%.
I'm not sure what alternatives to CloudWatch/RDS metrics you would need, or what you are looking for in an alternative for RDS storage metrics. What specifically is CloudWatch/RDS not providing you that you feel you need?