Add span for scheduler score

[gyliu513]

Sub task for #1483

[gyliu513]

/assign @mvanhorn

[mvanhorn]

Implemented in #1834 - adds a parent llm_d.epp.scoring span over the scorer chain plus per-scorer llm_d.epp.scorer.<type> child spans (identity, weight, candidate count, aggregate score max/avg), no per-endpoint attributes, and a no-op fast path when tracing is disabled.

[gyliu513]

The PR #1834 hasn't been merged yet.