fix(spanstats): set SkipMvccStats when node is designated for zero spans

[waterWang]

Description

When SkipApproxTotalStats is set and a node is the designated MVCC node for none of its spans, spansRequiringMvcc is computed as an empty list. An empty list is indistinguishable from the proto3 zero-value "not participating" case, so the receiving node's getLocalStats falls through and computes MVCC stats for all its spans — the opposite of the intended behavior.

Root cause

In getLocalStats, the skip decision is gated on len(req.SpansRequiringMvcc) > 0. When the list is empty (because this node is designated for zero spans), the guard fails silently and skipMvcc stays false, computing full MVCC for every span.

Fix

In nodeFn, when SkipApproxTotalStats is set and the computed spansRequiringMvcc is empty, set SkipMvccStats: true on the per-node request. This disambiguates the empty-list case: the receiving node sees SkipMvccStats == true and skips MVCC collection entirely.

Compatibility

SkipMvccStats is only set when SpansRequiringMvcc is empty, so verifySpanStatsRequest's mutual exclusivity check (len(SpansRequiringMvcc) > 0 && SkipMvccStats) is satisfied. The getLocalStats handler already checks req.SkipMvccStats before len(req.SpansRequiringMvcc), so this is backward compatible with older nodes.

Impact

When spansPerNode < nodes (e.g. a single large span whose ranges are spread across many nodes), the bug causes ~M nodes each to do a full-span meta scan and return TotalStats that gets summed into ApproximateTotalStats, violating the documented invariant:

When set ApproximateTotalStats will equal TotalStats since MVCC stats are collected from only one node rather than accumulated across all replicas.

Fixes #173200

Related: #161819, #138792

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