How to add ECS attributes to an instance using terraform
Terraform docs are pretty comprehensive, so if it’s not in the docs it’s probably not implemented yet.

However, they’re very responsive to feature requests posted as GitHub issues, particularly those that indicate that the feature is available via the AWS CLI (or more specifically, the AWS Go SDK). In my experience, feature requests I’ve raised have usually been available in a new AWS provider release within a month or so.

I’d suggest doing up a new issue here:
https://github.com/terraform-providers/terraform-provider-aws/issues

(I did do a quick search to see if ECS attributes have been raised before and couldn’t see any at a quick glance, but you may like to as well!)