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* Fix out-of-date inline flag reference syntax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/arp_linux.go

@@ -30,14 +30,14 @@ type arpCollector struct {
 }
 
 func init() {
-	Factories["arp"] = NewARPCollector
+	registerCollector("arp", defaultEnabled, NewARPCollector)
 }
 
 // NewARPCollector returns a new Collector exposing ARP stats.
 func NewARPCollector() (Collector, error) {
 	return &arpCollector{
 		entries: prometheus.NewDesc(
-			prometheus.BuildFQName(Namespace, "arp", "entries"),
+			prometheus.BuildFQName(namespace, "arp", "entries"),
 			"ARP entries by device",
 			[]string{"device"}, nil,
 		),