[ECS] [request]: Include Custom Metrics in Service Autoscaling UI

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**Tell us about your request**
What do you want us to build?
Enable ECS Service Autoscaling creation and edit menu, to show other metrics besides the Predefined ones.

- ALBRequestCountPerTarget
- ECSServiceAverageMemoryUtilization
- ECSServiceAverageCPUUtilization

**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?**

When creating a Scaling Policy via CFN or CLI, if you edit that policy in the console, only the predefined metrics are shown

**Are you currently working around this issue?**
Modifying via CFN or CLI

**Additional context**

**Attachments**
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Start with the ECS Service Autoscaling creation and edit menus, then compare policies created through CloudFormation or the CLI with what the console displays. Check the three listed metrics—ALBRequestCountPerTarget, ECSServiceAverageMemoryUtilization, and ECSServiceAverageCPUUtilization—and treat completion as making them available in both menus.

Written by the indexing model from the issue text.