Missing values For SageMaker Endpoint Invocation Metrics

Hi, I've deployed a custom model container as Sagemaker Async Endpoint which works fine so far. It's callable and provides results as expected. I now want to apply a TargetTrackingScalingPolicy but some of the metrics specified here seem to be not reported, they're also not visible in CloudWatch, e.g Invocations and InvocationsPerInstance. On the other hand, metrics like ModelLatency, TotalProcessingTime and TimeInBacklog show reasonable values.

I guess due to these values missing e.g. applying this policy config doesn't have any effect either:

TargetTrackingScalingPolicyConfiguration={
    "TargetValue": 2.0,  # The target value for the metric. - here the metric is - SageMakerVariantInvocationsPerInstance
    "CustomizedMetricSpecification": {
        "MetricName": "InvocationsPerInstance",
        "Namespace": "AWS/SageMaker",
        "Dimensions": [{"Name": "EndpointName", "Value": endpoint_name}],
        "Statistic": "Average",
    },
    "ScaleInCooldown": 60,
    "ScaleOutCooldown": 60
},

Thanks a lot for any advice!