Fix nil pointer exception in kubeletstats receiver (#404)

For the pods that are just created, some stats can be absent. And it causes crashing the otel agent with nil pointer exception.

receiver/kubeletstatsreceiver/kubelet/utils.go

@@ -59,5 +59,7 @@ func labels(labels map[string]string, descriptions map[string]string) (
 }
 
 func applyLabels(metric *metricspb.Metric, attrs map[string]string) {
-	metric.MetricDescriptor.LabelKeys, metric.Timeseries[0].LabelValues = labels(attrs, nil)
+	if metric != nil {
+		metric.MetricDescriptor.LabelKeys, metric.Timeseries[0].LabelValues = labels(attrs, nil)
+	}
 }