Skip collection stats when disabled in mongodb input (#6364)

plugins/inputs/mongodb/README.md

@@ -13,8 +13,10 @@
 
   ## When true, collect per database stats
   # gather_perdb_stats = false
+
   ## When true, collect per collection stats
   # gather_col_stats = false
+
   ## List of db where collections stats are collected
   ## If empty, all db are concerned
   # col_stats_dbs = ["local"]

plugins/inputs/mongodb/mongodb.go

@@ -42,8 +42,10 @@ var sampleConfig = `
 
   ## When true, collect per database stats
   # gather_perdb_stats = false
+
   ## When true, collect per collection stats
   # gather_col_stats = false
+
   ## List of db where collections stats are collected
   ## If empty, all db are concerned
   # col_stats_dbs = ["local"]
@@ -177,6 +179,7 @@ func (m *MongoDB) gatherServer(server *Server, acc telegraf.Accumulator) error {
 func init() {
 	inputs.Add("mongodb", func() telegraf.Input {
 		return &MongoDB{
+			ColStatsDbs: []string{"local"},
 			mongos:      make(map[string]*Server),
 		}
 	})

plugins/inputs/mongodb/mongodb_server.go

@@ -227,10 +227,14 @@ func (s *Server) gatherData(acc telegraf.Accumulator, gatherDbStats bool, gather
 			authLogLevel(err), err)
 	}
 
-	collectionStats, err := s.gatherCollectionStats(colStatsDbs)
+	var collectionStats *ColStats
+	if gatherColStats {
+		stats, err := s.gatherCollectionStats(colStatsDbs)
 		if err != nil {
 			return err
 		}
+		collectionStats = stats
+	}
 
 	dbStats := &DbStats{}
 	if gatherDbStats {

plugins/inputs/mongodb/mongostat.go

@@ -961,8 +961,8 @@ func NewStatLine(oldMongo, newMongo MongoStatus, key string, all bool, sampleSec
 		}
 	}
 
-	newColStats := *newMongo.ColStats
-	for _, col := range newColStats.Collections {
+	if newMongo.ColStats != nil {
+		for _, col := range newMongo.ColStats.Collections {
 			colStatsData := col.ColStatsData
 			// mongos doesn't have the db key, so setting the db name
 			if colStatsData.Collection == "" {
@@ -980,6 +980,7 @@ func NewStatLine(oldMongo, newMongo MongoStatus, key string, all bool, sampleSec
 			}
 			returnVal.ColStatsLines = append(returnVal.ColStatsLines, *colStatLine)
 		}
+	}
 
 	// Set shard stats
 	newShardStats := *newMongo.ShardStats