kv: add debugging obs for TestFlowControlSendQueueRangeFeed

I haven't been able to repro this. The test has failed twice in the
last couple of days, so hopefully we'll be able to see why the rangefeed
isn't ending up on n3 the next time this fails.

Closes cockroachdb#156064

Epic: none

Release note: None

Author: arulajmani

pkg/kv/kvclient/kvcoord/dist_sender_rangefeed.go

@@ -623,6 +623,7 @@ func newTransportForRange(
 		return nil, err
 	}
 	replicas.OptimizeReplicaOrder(ctx, ds.st, ds.nodeIDGetter(), ds.healthFunc, ds.latencyFunc, ds.locality)
+	log.VEventf(ctx, 2, "replica order for rangefeed transport is: %s", replicas)
 	opts := SendOptions{class: defRangefeedConnClass}
 	return ds.transportFactory(opts, replicas), nil
 }

pkg/kv/kvserver/flow_control_integration_test.go

@@ -3220,6 +3220,7 @@ func TestFlowControlSendQueueRangeSplitMerge(t *testing.T) {
 func TestFlowControlSendQueueRangeFeed(t *testing.T) {
 	defer leaktest.AfterTest(t)()
 	defer log.Scope(t).Close(t)
+	testutils.SetVModule(t, "dist_sender_mux_rangefeed=2,dist_sender_rangefeed=2")
 
 	// rangeFeed will create a rangefeed suitable for testing. It will start a
 	// rangefeed and return a function that can be used to stop it.