return metrics when grouping by category in stats_v2

[matejminar]

Currently, we query metrics in stats_v2 if a category=metrics is passed as a filter. Code. However there are cases where we want to also group by category. When that happens, results for other categories should be served from outcomes table (current behaviour) but metrics category should also be added.

This is not high priority but it will make the entire request response sequence much cleaner in FE