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* Fix out-of-date inline flag reference syntax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

@@ -30,7 +30,7 @@ const (
 type meminfoCollector struct{}
 
 func init() {
-	Factories["meminfo"] = NewMeminfoCollector
+	registerCollector("meminfo", defaultEnabled, NewMeminfoCollector)
 }
 
 // NewMeminfoCollector returns a new Collector exposing memory stats.
@@ -49,7 +49,7 @@ func (c *meminfoCollector) Update(ch chan<- prometheus.Metric) error {
 	for k, v := range memInfo {
 		ch <- prometheus.MustNewConstMetric(
 			prometheus.NewDesc(
-				prometheus.BuildFQName(Namespace, memInfoSubsystem, k),
+				prometheus.BuildFQName(namespace, memInfoSubsystem, k),
 				fmt.Sprintf("Memory information field %s.", k),
 				nil, nil,
 			),