Add benchmark search tracing

[Roman Schejbal (romanschejbal)]

• Does this PR have a descriptive title that could go in our release notes?
• Does this PR add any new dependencies?
• Does this PR modify any existing APIs?
• Is the change to the API backwards compatible?
• Should this result in any changes to our documentation, either updating existing docs or adding new ones?

Reference Issues/PRs

N/A

What does this implement/fix? Briefly explain your changes.

Adds opt-in diagnostic tracing for disk search under the benchmark-search-tracing feature.

The instrumentation records coarse spans for query preparation, graph search, and reranking, plus aggregate fields for comparisons, hops, beam expansions, neighbors considered, traversal I/O, vertex read/materialization time, PQ computation time, and success/error outcome. The feature is disabled by default and compiles to no-op metric storage when not enabled.

The tracing wrapper is integrated with the current indexed-vector search path and is intended for controlled diagnostic benchmarks.

Any other comments?

This is a draft to gather feedback on span names, aggregation boundaries, and disabled-path overhead before considering broader use.

Validation:

• cargo fmt --all -- --check
• cargo check -p diskann-disk --features benchmark-search-tracing
• cargo test -p diskann-disk --lib --features benchmark-search-tracing (252 passed)
• cargo clippy -p diskann-disk --features benchmark-search-tracing -- -D warnings

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