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* Fix out-of-date inline flag reference syntax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

@@ -36,7 +36,7 @@ type netDevCollector struct {
 }
 
 func init() {
-	Factories["netdev"] = NewNetDevCollector
+	registerCollector("netdev", defaultEnabled, NewNetDevCollector)
 }
 
 // NewNetDevCollector returns a new Collector exposing network device stats.
@@ -59,7 +59,7 @@ func (c *netDevCollector) Update(ch chan<- prometheus.Metric) error {
 			desc, ok := c.metricDescs[key]
 			if !ok {
 				desc = prometheus.NewDesc(
-					prometheus.BuildFQName(Namespace, c.subsystem, key),
+					prometheus.BuildFQName(namespace, c.subsystem, key),
 					fmt.Sprintf("Network device statistic %s.", key),
 					[]string{"device"},
 					nil,