[elbv2] support metric methods on IApplicationTargetGroup

ApplicationTargetGroup.FromTargetGroupAttributes returns an IApplicationTargetGroupProxy instead of IApplicationTargetGroup, as outlined in the documentation. This class cannot be cast to ApplicationTargetGroup

Reproduction Steps

var targetGroup = (ApplicationTargetGroup) ApplicationTargetGroup.FromTargetGroupAttributes(this, "TargetGroup", new TargetGroupAttributes {
    TargetGroupArn = props.TargetGroupArn,
    LoadBalancerArns = props.LoadBalancerArn
});
_metric = targetGroup.MetricHttpCodeTarget(props.StatusCode);

What did you expect to happen?

IApplicationTargetGroup is cast to ApplicationTargetGroup and returns HttpCode metric

What actually happened?

On cdk synth:

Unhandled exception. System.InvalidCastException: Unable to cast object of type 'Amazon.CDK.AWS.ElasticLoadBalancingV2.IApplicationTargetGroupProxy' to type 'Amazon.CDK.AWS.ElasticLoadBalancingV2.ApplicationTargetGroup'.

Environment

• CLI Version : 1.67.0
• Framework Version: 1.67.0
• Node.js Version: v12.19.0
• OS : OSX 10.14.6
• Language (Version): .NET Core 3.1.200

Other

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This is 🐛 Bug Report

[njlynch]

We can convert this to a feature request for exposing the metric methods for IApplicationTargetGroup. I can’t comment on or commit to a timeline when these will be added; however, pull requests are always appreciated!

[ysfaran]

In my case we separated resources from deployment. This means in our CI/CD pipeline we first have a job that creates all resources (ECS Cluster, ALB, TargetGroups, DynamoDB’s etc…). Secondly all ECS services are created and deployed to the ECS cluster.

Therefor we import the TargetGroup via ApplicationTargetGroup.fromTargetGroupAttributes during the second (deployment) step:

import elbv2 = require("aws-cdk-lib/aws-elasticloadbalancingv2")

const targetGroup = elbv2.ApplicationTargetGroup.fromTargetGroupAttributes(scope, `TargetGroup`, {
  targetGroupArn: myArn
})

And register the service + add a scaling metric for requests (scaleOnRequestCount):

import ecs = require("aws-cdk-lib/aws-ecs")

const service = new ecs.Ec2Service(this, "Service", { /* ... */ })

targetGroup.addTarget(service)

const scaling = service.autoScaleTaskCount({
  minCapacity: 1,
  maxCapacity: 4,
})

scaling.scaleOnRequestCount("RequestScaling", {
  requestsPerTarget: 1200,
  //  ERROR: Type 'IApplicationTargetGroup' is not assignable to type 'ApplicationTargetGroup'.
  targetGroup: targetGroup, 
})

Unfortunately ApplicationTargetGroup.fromTargetGroupAttributes only returns IApplicationTargetGroup and not ApplicationTargetGroup which ultimately leads to the error as seen in the snippet.

I also tried to cast it but it will fail during runtime then:

//  ERROR: Invalid resource label 'undefined/undefined' for predefined metric type ALBRequestCountPerTarget
targetGroup: props.targetGroup as elbv2.ApplicationTargetGroup

Is there any clean way to get around this as of now? How to get the actual ApplicationTargetGroup instance?