fix nil pointer to support mongo 4.4 (#32)

Author: hzhao-wish

collector/metrics.go

@@ -376,11 +376,13 @@ type PreloadStats struct {
 
 // Export exposes the preload stats.
 func (preloadStats *PreloadStats) Export(ch chan<- prometheus.Metric) {
-	metricsReplPreloadDocsNumTotal.Set(preloadStats.Docs.Num)
-	metricsReplPreloadDocsTotalMilliseconds.Set(preloadStats.Docs.TotalMillis)
+	if preloadStats != nil {
+		metricsReplPreloadDocsNumTotal.Set(preloadStats.Docs.Num)
+		metricsReplPreloadDocsTotalMilliseconds.Set(preloadStats.Docs.TotalMillis)
 
-	metricsReplPreloadIndexesNumTotal.Set(preloadStats.Indexes.Num)
-	metricsReplPreloadIndexesTotalMilliseconds.Set(preloadStats.Indexes.TotalMillis)
+		metricsReplPreloadIndexesNumTotal.Set(preloadStats.Indexes.Num)
+		metricsReplPreloadIndexesTotalMilliseconds.Set(preloadStats.Indexes.TotalMillis)
+	}
 }
 
 // StorageStats are the stats associated with the storage.
@@ -420,8 +422,8 @@ func (cursorStats *CursorStats) Export(ch chan<- prometheus.Metric) {
 
 // TTLStats are the stats with a ttl index
 type TTLStats struct {
-	DeletedDocuments float64          `bson:"deletedDocuments"`
-	Passes          float64           `bson:"passes"`
+	DeletedDocuments float64 `bson:"deletedDocuments"`
+	Passes           float64 `bson:"passes"`
 }
 
 // Export exports the ttl stats.
@@ -432,8 +434,8 @@ func (ttlStats *TTLStats) Export(ch chan<- prometheus.Metric) {
 
 // QueryStats are the stats with a ttl index
 type QueryStats struct {
-	PlanCacheSize   float64          `bson:"planCacheTotalSizeEstimateBytes"`
-	UpdateCount     float64          `bson:"updateOneOpStyleBroadcastWithExactIDCount"`
+	PlanCacheSize float64 `bson:"planCacheTotalSizeEstimateBytes"`
+	UpdateCount   float64 `bson:"updateOneOpStyleBroadcastWithExactIDCount"`
 }
 
 // Export exports the query stats.