fix: don't panic when return is nil

If the scrape fails, the return is nil for either project or regionList.
We should not try to access project.Quotas or regionList.Items in that
case.

Bonus: Even if the regionList call fails, we can still return the
project level results.

fixes #8

main.go

@@ -71,7 +71,7 @@ func (e *Exporter) scrape() (up float64, prj *compute.Project, rgl *compute.Regi
 	regionList, err := e.service.Regions.List(e.project).Do()
 	if err != nil {
 		log.Errorf("Failure when querying region quotas: %v", err)
-		return 0, nil, nil
+		return 0, project, nil
 	}
 
 	return 1, project, regionList
@@ -91,16 +91,20 @@ func (e *Exporter) Collect(ch chan<- prometheus.Metric) {
 
 	up, project, regionList := e.scrape()
 
-	for _, quota := range project.Quotas {
-		ch <- prometheus.MustNewConstMetric(limitDesc, prometheus.GaugeValue, quota.Limit, e.project, "", quota.Metric)
-		ch <- prometheus.MustNewConstMetric(usageDesc, prometheus.GaugeValue, quota.Usage, e.project, "", quota.Metric)
+	if project != nil {
+		for _, quota := range project.Quotas {
+			ch <- prometheus.MustNewConstMetric(limitDesc, prometheus.GaugeValue, quota.Limit, e.project, "", quota.Metric)
+			ch <- prometheus.MustNewConstMetric(usageDesc, prometheus.GaugeValue, quota.Usage, e.project, "", quota.Metric)
+		}
 	}
 
-	for _, region := range regionList.Items {
-		regionName := region.Name
-		for _, quota := range region.Quotas {
-			ch <- prometheus.MustNewConstMetric(limitDesc, prometheus.GaugeValue, quota.Limit, e.project, regionName, quota.Metric)
-			ch <- prometheus.MustNewConstMetric(usageDesc, prometheus.GaugeValue, quota.Usage, e.project, regionName, quota.Metric)
+	if regionList != nil {
+		for _, region := range regionList.Items {
+			regionName := region.Name
+			for _, quota := range region.Quotas {
+				ch <- prometheus.MustNewConstMetric(limitDesc, prometheus.GaugeValue, quota.Limit, e.project, regionName, quota.Metric)
+				ch <- prometheus.MustNewConstMetric(usageDesc, prometheus.GaugeValue, quota.Usage, e.project, regionName, quota.Metric)
+			}
 		}
 	}