`Metric` data object stores UI description

[JanWittler]

The Metric record stores the description that should be shown in the report viewer. This breaks the intended separation between data and UI. As such, we should refactor Metric to only contain an identifier of the type of metric (e.g. modeled as an enum) and load the according description in the report viewer based on that identifier.

After this refactoring, we should move the Messages class and all its resources to the CLI package and change the visibility to protected.