Add metrics to track split affinity ratio

[fulmicoton]

It would be interesting to know how often we end up picking the searcher with the best affinity.

[trinity-1686a]

i think this metric will be non-trivial to interpret. If it's at 100% or 0%, we are definitely doing perfectly, the worst case seemingly being 50%. But if every other split uses the 2nd best affinity, and does so consistency, we'll see 50% on that metric, yet 100% hit in practice.