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* Fix out-of-date inline flag reference syntax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

@@ -33,7 +33,7 @@ const (
 type netStatCollector struct{}
 
 func init() {
-	Factories["netstat"] = NewNetStatCollector
+	registerCollector("netstat", defaultEnabled, NewNetStatCollector)
 }
 
 // NewNetStatCollector takes and returns
@@ -72,7 +72,7 @@ func (c *netStatCollector) Update(ch chan<- prometheus.Metric) error {
 			}
 			ch <- prometheus.MustNewConstMetric(
 				prometheus.NewDesc(
-					prometheus.BuildFQName(Namespace, netStatsSubsystem, key),
+					prometheus.BuildFQName(namespace, netStatsSubsystem, key),
 					fmt.Sprintf("Statistic %s.", protocol+name),
 					nil, nil,
 				),