Add span-tracing to search path

[arkrishn94]

Add span tracing to disk-index search path. Goal should be to add granular span tracing so user-level telemetry provider (e.g. OpenTelemetry) can pick it up.

[coldbean-s]

Hi, I'd like to work on this issue. I'll first trace the disk-index search path and identify appropriate span boundaries, then submit a PR with the tracing instrumentation and tests.