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>

collector/scheduled_task.go

@@ -63,10 +63,6 @@ type ScheduledTask struct {
 
 type ScheduledTasks []ScheduledTask
 
-func init() {
-	registerCollector("scheduled_task", NewScheduledTask)
-}
-
 // NewScheduledTask ...
 func NewScheduledTask() (Collector, error) {
 	const subsystem = "scheduled_task"