scheduler_perf: do not override throughput labels

Throughput labels are currently initialized with a "Name" label.
So we need to append to the map instead of creating a new one.

Author: ingvagabund

test/integration/scheduler_perf/util.go

@@ -243,9 +243,7 @@ func (tc *throughputCollector) collect() []DataItem {
 			sum += tc.schedulingThroughputs[i]
 		}
 
-		throughputSummary.Labels = map[string]string{
-			"Metric": "SchedulingThroughput",
-		}
+		throughputSummary.Labels["Metric"] = "SchedulingThroughput"
 		throughputSummary.Data = map[string]float64{
 			"Average": sum / float64(length),
 			"Perc50":  tc.schedulingThroughputs[int(math.Ceil(float64(length*50)/100))-1],