feat: add upMetricName params to allow for up metric name customization

[BreizhHardware]

This pull request introduces a new configuration option to customize the name of the "up" metric in the HTTP metrics library, enhancing flexibility for users. The most important changes include updates to the documentation, the addition of the new option in the code, and its integration into the metric creation logic.

Documentation Updates:

• Added a description for the new upMetricName option in the README.md, which allows users to customize the name of the "up" metric. The default remains as up.

Code Enhancements:

• Introduced the upMetricName configuration option in the main function of src/index.js, defaulting to up if not provided.
• Updated the metric creation logic to use the customizable upMetricName instead of the hardcoded up, ensuring the new option is applied when creating the "up" metric.

Fix the issue #122