server/span_stats: fix SkipApproxTotalStats empty-ambiguity opt-out [fj4WqyCCw3C5ShR1RfB7MoBPTpkRrBFYP1uT35g3MvT]

[waterWang]

Fixes #173200

Problem

SkipApproxTotalStats (added in #161819, fixing #138792) is meant to ensure that during a SpanStats fan-out, only one node per span collects logical MVCC stats. The coordinator picks a designated MVCC node per span and populates SpansRequiringMvcc on that node's per-node request.

The mechanism relies on a non-empty SpansRequiringMvcc list to signal "restrict MVCC collection to this subset." But a replica-holding node that is designated for none of its spans receives an empty SpansRequiringMvcc, which is indistinguishable from "field not set" in proto3. Such a node falls through and computes MVCC stats for all of its spans — the opposite of the intended behavior.

In getLocalStats, the skip decision is gated on the list being non-empty:

skipMvcc := req.SkipMvccStats
if !skipMvcc && len(req.SpansRequiringMvcc) > 0 {
    _, requiresMvcc := spansRequiringMvcc[span.String()]
    skipMvcc = !requiresMvcc
}

Because the coordinator sets only SpansRequiringMvcc (never SkipMvccStats), a node with an empty computed list sees SkipMvccStats == false and len(SpansRequiringMvcc) == 0, and computes MVCC for every span it holds.

Fix

When SkipApproxTotalStats is set and the computed spansRequiringMvcc for a node is empty (the node is the designated MVCC collector for none of its spans), set SkipMvccStats: true on the per-node request so the receiving node skips MVCC collection entirely and reports physical bytes only.

This makes the per-node request unambiguous: SkipApproxTotalStats now guarantees exactly one node per span computes MVCC stats, and ApproximateTotalStats equals TotalStats regardless of the spans-to-nodes ratio — restoring the invariant documented in roachpb/span_stats.proto.

Test plan

• Logic is a single gating change in the coordinator fan-out (nodeFn).

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