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Split scheduler latency metric to fine-grained steps

This splits the summary metric we recently added into finer steps. It should be very useful for performance experiments.

/cc @wojtek-t 
fyi - @bsalamat @misterikkit 

Strictly speaking this is a breaking change, but since this metric was added only ~week ago I think it should fine (we should port this change to 1.11).

```release-note
Split 'scheduling_latency_seconds' metric into finer steps (predicate, priority, premption)
```