[ECS] [Request]: Cluster Auto Scaling: eliminate customer management of AmazonECSManaged Tag

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**Tell us about your request**

We would like for one of the following:

- the Cluster Auto Scaling service could utilize a tag which is not user modifiable (I think `aws:cloudformation` tags aren't modifiable? so perhaps `aws:ecs` tags could be handled in the same way?) thus eliminating the possibility of a user editing this tag and breaking Cluster Auto Scaling

**Which service(s) is this request for?**

ECS

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**

- an integration issue, where relying on tags (customer modifiable) for an integration is potentially problematic... where tags are created by AWS automatically but can then be removed by a user in a subsequent modification (such as a `terraform apply`)

**Are you currently working around this issue?**

We are (generally) applying tags using infra as code so the issue does no impact us - the reason we recognized this issue was an Engineer was building an ECS cluster and the tag was applied automatically. This created confusion as the tag was `AmazonECSManaged` not `AmazonECSManaged=true` (what they expected) and they believed this may have been the source of Cluster Auto Scaling not working.

**Additional context**

I think Cluster Auto Scaling is amazing (although it's still "a lot" in the customer's hands), but I believe this particular area is a rough edge.

**Attachments**

If you think you might have additional information that you'd like to include via an attachment, please do - we'll take a look. (Remember to remove any personally-identifiable information.)

Start by reviewing the ECS Cluster Auto Scaling and AmazonECSManaged tag behavior described in the issue, including how customer changes such as terraform apply can remove the tag. Done would require an AWS-supported approach that prevents customer modification or otherwise eliminates customer management of this integration tag.

Written by the indexing model from the issue text.