Tezt_cloud: Refactor Monitoring utilities

What

Parent MR: !18785 (merged)

Refactoring the following pieces of code and moving them in their own modules:

• metrics
• monitoring_app

Why

Currently, I will try to split the DAL scenario in self-contained chunks, and after this is finished, we can adapt the LAYER1 scenario to use the newly introduced utilities.

How

Manually testing the MR

Check with any tezt-cloud DAL scenario. This is just moving code from one place to another, so it should be a no-op.

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR