Replace --collectors.enabled with per-collector flags (#640)

* Move NodeCollector into package collector

* Refactor collector enabling

* Update README with new collector enabled flags

* Fix out-of-date inline flag reference syntax

* Use new flags in end-to-end tests

* Add flag to disable all default collectors

* Track if a flag has been set explicitly

* Add --collectors.disable-defaults to README

* Revert disable-defaults flag

* Shorten flags

* Fixup timex collector registration

* Fix end-to-end tests

* Change procfs and sysfs path flags

* Fix review comments

collector/cpu_dragonfly.go

@@ -88,14 +88,14 @@ type statCollector struct {
 }
 
 func init() {
-	Factories["cpu"] = NewStatCollector
+	registerCollector("cpu", defaultEnabled, NewStatCollector)
 }
 
 // NewStatCollector returns a new Collector exposing CPU stats.
 func NewStatCollector() (Collector, error) {
 	return &statCollector{
 		cpu: prometheus.NewDesc(
-			prometheus.BuildFQName(Namespace, "", "cpu"),
+			prometheus.BuildFQName(namespace, "", "cpu"),
 			"Seconds the cpus spent in each mode.",
 			[]string{"cpu", "mode"}, nil,
 		),