Ranking Metrics - Support segments

[FelipeAdachi]

Description

Currently, Ranking metrics don't support segments. We need to investigate why this isn't the case and, if possible, support it.

One reason (but possibly not the only reason) is the existence of non-mergeable metrics. Currently, the only non-mergeable metric is accuracy_k_.

• [x] I have reviewed the Guidelines for Contributing and the Code of Conduct.