outlet/kafka: make lag test more robust

The consumer may not have started when testing initial lag. Just try a
bit more.

outlet/kafka/functional_test.go

@@ -367,7 +367,7 @@ func TestKafkaLagMetric(t *testing.T) {
 	defer cluster.Close()
 
 	// Watch for autocommits to avoid relying on time
-	clusterCommitNotification := make(chan interface{})
+	clusterCommitNotification := make(chan any)
 	cluster.Control(func(request kmsg.Request) (kmsg.Response, error, bool) {
 		switch k := kmsg.Key(request.Key()); k {
 		case kmsg.OffsetCommit:
@@ -404,7 +404,7 @@ func TestKafkaLagMetric(t *testing.T) {
 	helpers.StartStop(t, c)
 
 	// Start a worker with a callback that blocks on a channel after receiving a message
-	workerBlockReceive := make(chan interface{})
+	workerBlockReceive := make(chan any)
 	defer close(workerBlockReceive)
 	c.StartWorkers(func(_ int, _ chan<- ScaleRequest) (ReceiveFunc, ShutdownFunc) {
 		return func(context.Context, []byte) error {
@@ -414,14 +414,24 @@ func TestKafkaLagMetric(t *testing.T) {
 	})
 
 	// No messages yet, no lag
-	time.Sleep(10 * time.Millisecond)
-	gotMetrics := r.GetMetrics("akvorado_outlet_kafka_", "consumergroup", "workers")
-	expected := map[string]string{
-		"consumergroup_lag_messages": "0",
-		"workers":                    "1",
-	}
-	if diff := helpers.Diff(gotMetrics, expected); diff != "" {
-		t.Fatalf("Metrics (-got, +want):\n%s", diff)
+	ctx, cancel := context.WithTimeout(t.Context(), time.Second)
+	defer cancel()
+	for {
+		gotMetrics := r.GetMetrics("akvorado_outlet_kafka_", "consumergroup", "workers")
+		expected := map[string]string{
+			"consumergroup_lag_messages": "0",
+			"workers":                    "1",
+		}
+		if diff := helpers.Diff(gotMetrics, expected); diff != "" {
+			select {
+			case <-ctx.Done():
+				t.Fatalf("Metrics (-got, +want):\n%s", diff)
+			default:
+			}
+			time.Sleep(10 * time.Millisecond)
+		} else {
+			break
+		}
 	}
 
 	// Send a single message, allow it to be processed
@@ -442,8 +452,8 @@ func TestKafkaLagMetric(t *testing.T) {
 	}
 
 	// The message was processed, there's no lag
-	gotMetrics = r.GetMetrics("akvorado_outlet_kafka_", "consumergroup", "received_messages_total")
-	expected = map[string]string{
+	gotMetrics := r.GetMetrics("akvorado_outlet_kafka_", "consumergroup", "received_messages_total")
+	expected := map[string]string{
 		"consumergroup_lag_messages":          "0",
 		`received_messages_total{worker="0"}`: "1",
 	}