[Feature Request]: Add Unit Test for Metrics Module

### Preflight Checklist

- [x] I have read the [Contributing Guidelines](https://github.com/EnAccess/OpenPAYGO-js/blob/main/CONTRIBUTING.md) for this project, if it exists.
- [x] I agree to follow the [Code of Conduct](https://github.com/EnAccess/OpenPAYGO-js/blob/main/CODE_OF_CONDUCT.md) that this project adheres to.
- [x] I have searched the [issue tracker](https://github.com/EnAccess/OpenPAYGO-js/issues) for a feature request that matches the one I want to file, without success.

### Problem Description

Once #43 is merged. It would be great to have unit tests for the metrics feature similar to the token module.

### Proposed Solution

Add a test file i.e `metrics.test.js` with unit tests for `MetricsRequestHandler`, `MetricsResponseHandler` and `AuthMethod`.

### Alternatives Considered

No alternative solution was considered.

### Additional Information

_No response_