perf(block): precompute OTEL for chunker hot paths

[levb]

Replace per-call attribute.String(...) allocation in every Slice/ReadAt/runFetch with precomputed metric.MeasurementOption values built once at package init.

• Stopwatch.end: build one attribute.NewSet and reuse across all three instrument calls (histogram, sum, count) instead of three separate metric.WithAttributes(kv...) allocations.

• New PrecomputeAttrs(kv...): builds a reusable MeasurementOption.

• New Stopwatch.Record(ctx, total, precomputedAttrs): zero per-call attribute allocation alternative to Success/Failure.

• Exported Success/Failure attribute vars for use with PrecomputeAttrs.

• Add precomputedAttrs struct with all Slice/fetch attribute combos.

• Package-level chunkerAttrs var built once at init.

• All timer.Success(ctx, n, attribute.String(...)) calls replaced with timer.Record(ctx, n, a.successFromCache) etc.

FullFetchChunker.Slice on cache hit (64 MiB cache, 4K blocks, 4 MiB chunks):

                         │    baseline     │         precomputed attrs          │
                         │     sec/op      │   sec/op     vs base              │
ChunkerSlice_CacheHit-16     20.57µ ± 2%   19.85µ ± 3%  -3.49% (p=0.009 n=6)

                         │    baseline     │         precomputed attrs              │
                         │      B/op       │     B/op      vs base                  │
ChunkerSlice_CacheHit-16    9.211Ki ± 0%   8.047Ki ± 0%  -12.64% (p=0.002 n=6)

                         │    baseline     │        precomputed attrs             │
                         │   allocs/op     │ allocs/op   vs base                  │
ChunkerSlice_CacheHit-16     16.000 ± 0%    4.000 ± 0%  -75.00% (p=0.002 n=6)

75% fewer allocations (16 → 4) and 13% less memory per Slice call on the NBD page fault hot path.

[dobrac]

Superseeds: #2236

[claude]

RecordRaw silently drops t.kv (the base attributes stored by Begin(kv ...)). All current call sites pass no args to Begin(), so this is fine today. But since RecordRaw is now exported, a future caller could do timer := factory.Begin(sandboxID, ...) and then call RecordRaw, silently losing those base dimensions from all three metric recordings without any compile-time or runtime warning.

Consider either: (a) merging t.kv into the precomputed option inside RecordRaw when t.kv is non-nil, or (b) keeping RecordRaw unexported and only exposing it through the typed Success/Failure methods.