[service] [request]: Target tracking scaling policy based on ALBRequestCountPerTarget becomes unusable in case of ECS blue green deployment.

### Community Note

* Please vote on this issue by adding a 👍 [reaction](https://blog.github.com/2016-03-10-add-reactions-to-pull-requests-issues-and-comments/) to the original issue to help the community and maintainers prioritize this request
* Please do not leave "+1" or "me too" comments, they generate extra noise for issue followers and do not help prioritize the request
* If you are interested in working on this issue or have submitted a pull request, please leave a comment

**Tell us about your request**
Target tracking scaling policy in ECS service autoscaling can be created based on ALBRequestCountPerTarget metric, however you can only select active target group associated with Load balancer. In an ECS blue green deployment situation this target group changes behind load balancer, old policy attached to blue becomes "insufficient data" as there is no active traffic in the blue target group when green is active. We tried to use two policies each for each target groups but that can't be possible as policy can be created only with active target group.

**Which service(s) is this request for?**
This could be Fargate, ECS

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
One target tracking policy should be able to handle both blue and green target groups even if its not active currently.

**Are you currently working around this issue?**
By attempting to create two policies each for each target groups.

**Additional context**
Anything else we should know?

**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 with the issue’s description of ECS service autoscaling, blue-green deployments, and the ALBRequestCountPerTarget metric. Clarify how a target-tracking policy could cover both blue and green target groups, then validate the behavior against the reported insufficient-data case. Done means one policy remains usable as traffic moves between target groups.

Written by the indexing model from the issue text.