kvserver: don't log "local QPS is below max threshold" by default

This log message is the 9th most voluminous across the CC fleet.
It's not surprising since most stores are idle.
There's no good reason to have it logged by default - it doesn't say
anything interesting.
Let's just keep it in traces.

Release note: None

pkg/kv/kvserver/store_rebalancer.go

@@ -391,8 +391,11 @@ func (sr *StoreRebalancer) ShouldRebalanceStore(ctx context.Context, rctx *Rebal
 	// We only bother rebalancing stores that are fielding more than the
 	// cluster-level overfull threshold of QPS.
 	if !(rctx.LocalDesc.Capacity.QueriesPerSecond > rctx.QPSMaxThreshold) {
-		log.KvDistribution.Infof(
-			ctx,
+		// Since the lack of activity is the most common case, we don't
+		// log externally by default. Only show the details when
+		// requested by log config or when looking at traces.
+		log.KvDistribution.VEventf(
+			ctx, 1,
 			"local QPS %.2f is below max threshold %.2f (mean=%.2f); no rebalancing needed",
 			rctx.LocalDesc.Capacity.QueriesPerSecond, rctx.QPSMaxThreshold, rctx.allStoresList.CandidateQueriesPerSecond.Mean)
 		return false