This extends the check so it can hold any arbitrary data. That is then
used in the generate report command, that finds all checks tagged with
report tag, executes them and gather their data for the final YAML
creation.

This commit also includes two examples of metrics that uses database as
the Data source.

With this new child class, report checks can share helpers easily. Also
the parent Check class, which is used elsewhere, does not need to be
extended by the data storage.

Earlier, all Reports were also considered Checks, so they showed up in
health checks. This change now fully isolates Reports and Checks while
all the code is shared.