fix: apply locality filter in statsForSpan to prevent ApproximateTotalStats over-count

[waterWang]

Summary

SpanStats.ApproximateTotalStats is intended to approximate the post-replication (physical) MVCC size of a span — roughly logical_size × replication_factor. Instead, it scales with the number of nodes the span's ranges live on, not the replication factor. For a span that occupies more nodes than RF, ApproximateTotalStats over-reports by numNodesContacted / RF.

Root cause

In statsForSpan, every fanned-out node iterates all range descriptors overlapping the span and collects MVCC stats for every range — regardless of whether the node holds a replica of that range. The coordinator's collectSpanStatsResponses then sums each node's full logical TotalStats into ApproximateTotalStats:

res.SpanToStats[spanStr].ApproximateTotalStats.Add(spanStats.TotalStats)

Since each node returns the full logical size for all ranges, the sum equals numNodes × logical instead of RF × logical. The bug is masked for spans confined to exactly RF nodes (where numNodes == RF) and only appears once a span is wide enough to live on more than RF nodes.

Fix

Apply a locality filter in statsForSpan: only collect MVCC stats for ranges where the current node holds a replica. This makes each node's contribution its actual share of the span, so the coordinator's sum across nodes yields the true replicated (RF × logical) total.

The metadata (RangeCount, ReplicaCount, StoreIDs) is still recorded for every range so the coordinator retains complete span-wide information.

Fixes #173182

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