Get AvailableIpAddressCount from aws_subnet Data Source

[ghost]

This issue was originally opened by @MMarulla as hashicorp/terraform#23781. It was migrated here as a result of the provider split. The original body of the issue is below.

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Current Terraform Version

Terraform v0.12.6
+ provider.aws v2.33.0

Use-cases

Assign new instances to the least subscribed subnet in a VPC.

Proposal

The aws_subnet data source relies on the underlying EC2 describe-subnets API. One of the values returned by that call is the number of available IP addresses in the subnet:

AvailableIpAddressCount -> (integer)
The number of unused private IPv4 addresses in the subnet. The IPv4 addresses for any stopped instances are considered unavailable.

If that field were included in the result attributes of aws_subnet data source, a decision could be made in code as to which subnet to place a new instance.

Note that available-ip-address-count can be passed as a filter value in the aws_subnet data source, but that does not help in finding the least subscribed subnet without repeated trail-and-error calls.

References

https://www.terraform.io/docs/providers/aws/d/subnet.html
https://docs.aws.amazon.com/cli/latest/reference/ec2/describe-subnets.html

[tmarlok88]

#13554

[justinretzolk]

Closed by #13554

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