Generate HealthyHostCount metric for retrieved NetworkTargetGroup

[marcinpz]

Hi,
I see there is method NetworkTargetGroup.metricHealthyHostCount() to generate HealthyHostCount metric but when I use elbv2.NetworkTargetGroup.fromTargetGroupAttributes() to obtain existing instance then it gives object of type INetworkTargetGroup which does not implement metricHealthyHostCount. I can use sth like this:

new cloudwatch.Metric({
            // https://docs.aws.amazon.com/elasticloadbalancing/latest/application/load-balancer-cloudwatch-metrics.html
            namespace: 'AWS/ApplicationELB',
            metricName: 'HealthyHostCount',
 ...

but previous method looks better/is shorter/don't need to specify some params.
And sth like this (casting) does not work:

    const metric = (targetGroup as elbv2.NetworkTargetGroup).metricHealthyHostCount();
                                                                 ^
TypeError: targetGroup.metricHealthyHostCount is not a function

[pahud]

metricHealthHostCount() is actually a construct method of NetworkTargetGroup L2 construct class and only available when you create a new NetworkTargetGroup object.

[marcinpz]

I see but don't understand why?! Why I cant generate metric for existing NTG easy way??