searcher: add ObservationContext to diskcache

Created by: keegancsmith

ObservationContext on diskcache was introduced in #28922. This means for about 5 months we haven't had metrics for it on searcher. This commit is the most minimal introduction for it. We don't yet integrate ObservationContext into searcher.

I considered just constructing an ObservationContext in Start, but then the prometheus registry would panic in tests due to multiple constructions of it.

Test Plan: go test and running an unindexed search locally.