[EKS] [request]: CloudWatch metric for EKS managed nodegroup status

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**Tell us about your request**
Expose EKS managed nodegroup status (active or degraded) as a CloudWatch metric which can be used to create alarm

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

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
Currently it is possible to get status (active or degraded) of EKS managed nodegroup via console, describe-nodegroup CLI or DescribeNodegroup API. These methods also reveal health issues if any with the node group as described on https://github.com/awsdocs/amazon-eks-user-guide/blob/master/doc_source/troubleshooting.md#troubleshoot-managed-node-groups. But it will be good to have this exposed as a CloudWatch metric (may be in Container Insights) so that an alarm and notification can be created based on this.

**Are you currently working around this issue?**
No

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

Start by reviewing the existing EKS status sources named in the issue: the console, describe-nodegroup CLI, DescribeNodegroup API, and the managed node group troubleshooting guidance. The requested outcome is a CloudWatch metric representing active or degraded status that can drive an alarm and notification, but the issue does not identify repository files or tests.

Written by the indexing model from the issue text.