FreeStorageSpace

 AWS_DEFAULT_REGION="$REGION" aws cloudwatch get-metric-statistics \
 --namespace AWS/RDS --metric-name FreeStorageSpace \
 --start-time $START --end-time $END --period 300 \
 --statistics Average \
 --dimensions "Name=YOUR_DBInstanceIdentifier, Value=${INSTANCE_NAME}"

aws cloudwatch get-metric-statistics             \
               --metric-name FreeStorageSpace    \
               --start-time 2022-07-01T23:00:00Z \ 
               --end-time 2022-07-30T23:00:00Z   \
               --period 3600                     \
               --namespace AWS/RDS               \
               --statistics Average              \  
               --dimensions Name=DBInstanceIdentifier,Value=${INSTANCE_NAME}

See also

• aws cloudwatch [list-dashboards | create-log-group | put-metric-alarm | describe-alarms | set-alarm-state | list-metrics | get-metric-statistics ], CloudwatchLogsGroup