[dnm] kvserver: 10% synthetic reproposals

Hoping to shake out something relevant for
cockroachdb#97102 or
cockroachdb#97141.

Not intended for merge. Can maybe turn this into a metamorphic var down
the road. For now running manually via `./experiment.sh`.

To date, it hasn't produced anything. For the first runs, I ~immediately
got a closed timestamp regression. This was puzzling, but then also
disappeared, so I think my OSX clock might have been adjusting over
those few minutes.

Epic: none
Release note: None

experiment.sh

@@ -0,0 +1,9 @@
+#!/bin/bash
+set -euxo pipefail
+./dev build short
+roachprod wipe local
+roachprod put local cockroach
+roachprod start local
+roachprod sql local:1 -- -e 'set cluster setting kv.range_split.load_qps_threshold = 9999999'
+roachprod ssh local:1 -- ./cockroach workload init kv --splits 1
+roachprod ssh local:1 -- ./cockroach workload run kv --read-percent 0 {pgurl} --concurrency 512

pkg/kv/kvserver/replica_proposal_buf.go

@@ -12,6 +12,7 @@ package kvserver
 
 import (
 	"context"
+	"math/rand"
 	"sync"
 	"sync/atomic"
 
@@ -766,6 +767,9 @@ func (b *propBuf) allocateLAIAndClosedTimestampLocked(
 	if !p.Request.IsSingleRequestLeaseRequest() {
 		b.assignedLAI++
 		lai = b.assignedLAI
+		if rand.Intn(10) == 5 && lai > 1234 {
+			lai = 1234 // to spot it visually
+		}
 	}
 
 	if filter := b.testing.leaseIndexFilter; filter != nil {