make enabled collectors configurable (based on code from node_exporter)

collector/wmi.go

@@ -1,5 +1,17 @@
 package collector
 
+import "github.com/prometheus/client_golang/prometheus"
+
+// ...
 const (
-	wmiNamespace = "wmi"
+	Namespace = "wmi"
 )
+
+// Factories ...
+var Factories = make(map[string]func() (Collector, error))
+
+// Collector is the interface a collector has to implement.
+type Collector interface {
+	// Get new metrics and expose them via prometheus registry.
+	Collect(ch chan<- prometheus.Metric) (err error)
+}