kv: log warnings on merge txn starvation

Informs cockroachdb#125779.

This commit makes logging more verbose when a merge txn appears to be
starved. This would help with debugging cockroachdb#125779, which appears to be a
long-standing rare test flake.

Release note: None

pkg/kv/kvserver/replica_command.go

@@ -931,11 +931,15 @@ func (r *Replica) AdminMerge(
 	// is finicky and only allows disabling pipelining before any operations have
 	// been sent, even in prior epochs. Calling DisablePipelining() on a restarted
 	// transaction yields an error.
-	for {
+	for attempt := 1; ; attempt++ {
 		txn := kv.NewTxn(ctx, r.store.DB(), r.NodeID())
 		err := runMergeTxn(txn)
 		if err != nil {
-			log.VEventf(ctx, 2, "merge txn failed: %s", err)
+			if attempt < 3 {
+				log.VEventf(ctx, 2, "merge txn failed: %s", err)
+			} else {
+				log.Warningf(ctx, "merge txn failed (attempt %d): %s", attempt, err)
+			}
 			if rollbackErr := txn.Rollback(ctx); rollbackErr != nil {
 				log.VEventf(ctx, 2, "merge txn rollback failed: %s", rollbackErr)
 			}