Add a `MetricReporter`

[WarmCyan]

Curifactory should provide a MetricReporter that's handled differently during report generation - All metrics should be added to a table(s) at the top of the reportables section (removing the need for aggregate stages in very simple cases), and curifactory could also specially handle these to add these metrics (which need to be single floats perhaps) to the run_info and database, leaving open the opportunity to render them on the reports index as well, or even allow printing them at the CLI like DVC does.

Some thought should be put into the relationship between artifacts and reportables - is there an easy way to programmatically get the reportable outside of just the report? (e.g. if I report a metric but never return it, is it still possible to easily access it from a script?)