Only iniate collectors once

When /metrics is called for specific collectors, the collectors are
initialed every time. Which means that we spend a lot of time
re-initiating the same collectors again and again. Especially, some
collectors make the assumptions that that are initiated once - e.g.
systemd collector generates info message upon initiation.

Signed-off-by: Julien Pivotto <roidelapluie@inuits.eu>

collector/collector.go

@@ -50,9 +50,11 @@ const (
 )
 
 var (
-	factories        = make(map[string]func(logger log.Logger) (Collector, error))
-	collectorState   = make(map[string]*bool)
-	forcedCollectors = map[string]bool{} // collectors which have been explicitly enabled or disabled
+	factories              = make(map[string]func(logger log.Logger) (Collector, error))
+	initiatedCollectorsMtx = sync.Mutex{}
+	initiatedCollectors    = make(map[string]Collector)
+	collectorState         = make(map[string]*bool)
+	forcedCollectors       = map[string]bool{} // collectors which have been explicitly enabled or disabled
 )
 
 func registerCollector(collector string, isDefaultEnabled bool, factory func(logger log.Logger) (Collector, error)) {
@@ -115,14 +117,21 @@ func NewNodeCollector(logger log.Logger, filters ...string) (*NodeCollector, err
 		f[filter] = true
 	}
 	collectors := make(map[string]Collector)
+	initiatedCollectorsMtx.Lock()
+	defer initiatedCollectorsMtx.Unlock()
 	for key, enabled := range collectorState {
 		if *enabled {
-			collector, err := factories[key](log.With(logger, "collector", key))
-			if err != nil {
-				return nil, err
-			}
-			if len(f) == 0 || f[key] {
+			if collector, ok := initiatedCollectors[key]; ok {
 				collectors[key] = collector
+			} else {
+				collector, err := factories[key](log.With(logger, "collector", key))
+				if err != nil {
+					return nil, err
+				}
+				if len(f) == 0 || f[key] {
+					collectors[key] = collector
+					initiatedCollectors[key] = collector
+				}
 			}
 		}
 	}