aws-applicationautoscaling: Target Tracking Scaling Policy does not support account for custom metric

### Describe the bug

Target Scaling Policy does not return the account information from the metric so cross account metric cannot be used.

https://github.com/aws/aws-cdk/blob/244c02de488a9dfe7dad9eca1e9bb301863216bc/packages/aws-cdk-lib/aws-applicationautoscaling/lib/target-tracking-scaling-policy.ts#L170

### Regression Issue

- [ ] Select this option if this issue appears to be a regression.

### Last Known Working CDK Library Version

_No response_

### Expected Behavior

The renderCustomMetric function the target tracking policy should return the account from the metric as well

https://github.com/aws/aws-cdk/blob/244c02de488a9dfe7dad9eca1e9bb301863216bc/packages/aws-cdk-lib/aws-applicationautoscaling/lib/target-tracking-scaling-policy.ts#L162C10-L162C28

### Current Behavior

The account information is never fetched from the metric so it default to the current account when creating it

### Reproduction Steps

Create a Target Tracking Policy using a Custom Metric and specify the a different account for the metric and it will be ignored

### Possible Solution

_No response_

### Additional Information/Context

_No response_

### AWS CDK Library version (aws-cdk-lib)

2.223.0

### AWS CDK CLI version

2.1031.2

### Node.js Version

22.9.0

### OS

Amazon Linux 2023

### Language

Python

### Language Version

3.11

### Other information

_No response_

Start in packages/aws-cdk-lib/aws-applicationautoscaling/lib/target-tracking-scaling-policy.ts at renderCustomMetric and inspect how the custom metric account is handled. Reproduce the cross-account metric case and run the relevant aws-applicationautoscaling tests; done means the synthesized target tracking policy preserves the metric's specified account instead of defaulting to the current account.

Written by the indexing model from the issue text.