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* Fix out-of-date inline flag reference syntax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

@@ -27,7 +27,7 @@ type xfsCollector struct {
 }
 
 func init() {
-	Factories["xfs"] = NewXFSCollector
+	registerCollector("xfs", defaultEnabled, NewXFSCollector)
 }
 
 // NewXFSCollector returns a new Collector exposing XFS statistics.
@@ -179,7 +179,7 @@ func (c *xfsCollector) updateXFSStats(ch chan<- prometheus.Metric, s *xfs.Stats)
 
 	for _, m := range metrics {
 		desc := prometheus.NewDesc(
-			prometheus.BuildFQName(Namespace, subsystem, m.name),
+			prometheus.BuildFQName(namespace, subsystem, m.name),
 			m.desc,
 			labels,
 			nil,