roachtest: metamorphically enable lead_for_global_reads_auto_tune

This commit metamorphically enables lead_for_global_reads_auto_tune
for follower reads roachtests.

Release note: none
Epic: none

Author: wenyihu6

pkg/cmd/roachtest/tests/follower_reads.go

@@ -219,7 +219,19 @@ func runFollowerReadsTest(
 	require.NoError(t, func() error {
 		db := c.Conn(ctx, l, 1)
 		defer db.Close()
-		err := enableIsolationLevels(ctx, t, db)
+		err := enableClosedTsAutoTune(ctx, rng, db, t)
+		if err != nil && strings.Contains(err.Error(), "unknown cluster setting") {
+			// Versions v25.1 and earlier do not support these cluster settings and
+			// should ignore them. The cluster will continue operating normally, with
+			// old-version nodes ignoring the settings and newer-version nodes
+			// continuing to run. Auto-tuning of closed timestamps should only start
+			// taking effect when the entire cluster has been upgraded to v25.2.
+			err = nil
+		}
+		if err != nil {
+			return err
+		}
+		err = enableIsolationLevels(ctx, t, db)
 		if err != nil && strings.Contains(err.Error(), "unknown cluster setting") {
 			// v23.1 and below does not have these cluster settings. That's fine, as
 			// all isolation levels will be transparently promoted to "serializable".
@@ -1080,3 +1092,20 @@ func enableTenantMultiRegion(l *logger.Logger, r *rand.Rand, h *mixedversion.Hel
 	err := setTenantSetting(l, r, h, setting, true)
 	return errors.Wrapf(err, "setting %s", setting)
 }
+
+// enableClosedTsAutoTune metamorphically enables closed timestamp auto-tuning.
+func enableClosedTsAutoTune(ctx context.Context, rng *rand.Rand, db *gosql.DB, t test.Test) error {
+	if rng.Intn(2) == 0 {
+		for _, cmd := range []string{
+			`SET CLUSTER SETTING kv.closed_timestamp.lead_for_global_reads_auto_tune.enabled = 'true';`,
+			`SET CLUSTER SETTING kv.closed_timestamp.policy_refresh_interval = '5s';`,
+			`SET CLUSTER SETTING kv.closed_timestamp.policy_latency_refresh_interval = '4s';`,
+		} {
+			if _, err := db.ExecContext(ctx, cmd); err != nil {
+				return err
+			}
+		}
+		t.L().Printf("metamorphically enabled closed timestamp auto-tuning")
+	}
+	return nil
+}