feat: Remove init functions from collectors

Behaviour of init functions has been centralised in `collector/init.go`,
and can be called during exporter startup. This allows the exporter to
control the timing of collector initialisation, rather than relying on
the import & `init()` method.

This should reduce unexpected behaviour arising from the use of
`init()`, such as #551.

Signed-off-by: Ben Reedy <breed808@breed808.com>

exporter.go

@@ -333,6 +333,9 @@ func main() {
 
 	initWbem()
 
+	// Initialize collectors before loading
+	collector.RegisterCollectors()
+
 	collectors, err := loadCollectors(*enabledCollectors)
 	if err != nil {
 		log.Fatalf("Couldn't load collectors: %s", err)