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* Fix out-of-date inline flag reference syntax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

@@ -23,14 +23,14 @@ type interruptsCollector struct {
 }
 
 func init() {
-	Factories["interrupts"] = NewInterruptsCollector
+	registerCollector("interrupts", defaultDisabled, NewInterruptsCollector)
 }
 
 // NewInterruptsCollector returns a new Collector exposing interrupts stats.
 func NewInterruptsCollector() (Collector, error) {
 	return &interruptsCollector{
 		desc: typedDesc{prometheus.NewDesc(
-			Namespace+"_interrupts",
+			namespace+"_interrupts",
 			"Interrupt details.",
 			interruptLabelNames, nil,
 		), prometheus.CounterValue},