aws_applicationautoscaling: Error: Only direct metrics are supported for Target Tracking. Use Step Scaling or supply a Metric object.

### Describe the bug

I would like to use a `MathExpression` for custom metric in `TargetTrackingScalingPolicy`, but I got this error:
> Only direct metrics are supported for Target Tracking. Use Step Scaling or supply a Metric object.

checking the code [here](https://github.com/aws/aws-cdk/blob/v1-main/packages/@aws-cdk/aws-applicationautoscaling/lib/target-tracking-scaling-policy.ts#L133), it only checks for `metricStat` not `mathExpression`.

### Expected Behavior

Should allow to define math expression for Target Tracking.

### Current Behavior

Only direct metrics are allowed

### Reproduction Steps

Create Target Tracking using math expression

### Possible Solution

_No response_

### Additional Information/Context

_No response_

### CDK CLI Version

2.27.0

### Framework Version

_No response_

### Node.js Version

16.15.0

### OS

Debian 10

### Language

Python

### Language Version

_No response_

### Other information

_No response_

Read packages/@aws-cdk/aws-applicationautoscaling/lib/target-tracking-scaling-policy.ts around line 133, where the implementation checks metricStat but not mathExpression. Reproduce a TargetTrackingScalingPolicy using a MathExpression and verify the AWS Target Tracking requirements. Done means valid MathExpression metrics are accepted without the direct-metric error and the existing behavior for direct metrics remains intact.

Written by the indexing model from the issue text.