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

@@ -51,6 +51,8 @@ const (
 
 var (
 	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
 )
@@ -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 {
+			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
+				}
 			}
 		}
 	}