Track index-type latency metrics in Prometheus

[sparknack]

Background

Knowhere latency metrics currently do not distinguish between different index types, which makes it hard to compare search/build/load/range-search latency across FLAT, HNSW, IVF, DISKANN, sparse indexes, and other index families.

Goal

Add Prometheus latency metrics that can be broken down by index_type while preserving the existing Cardinal metric behavior.

Scope

• expose index_type as a label on Knowhere latency metrics
• keep the metric names as build_latency, load_latency, search_latency, and range_search_latency
• retain Cardinals existing metric compatibility
• ensure the metric lookup path is concurrency-safe

Notes

This is being addressed by PR #1577.