Steps to reproduce:

1. Configure RDS integration
2. Try to see any value for aws.rds.free_storage_space being inserted in Zabbix

Result:
No values found

Expected:
aws.rds.free_storage_space values present

Reading the code of the integration on git and AWS RDS documentation for Cloudwatch metrics, I presume that this line:
{{}}

'FreeStorageSpace:Percent',

should be rewritten as:
{{}}

'FreeStorageSpace:Bytes',