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/process.go

@@ -15,10 +15,6 @@ import (
 	"github.com/yusufpapurcu/wmi"
 )
 
-func init() {
-	registerCollector("process", newProcessCollector, "Process")
-}
-
 var (
 	processWhitelist = kingpin.Flag(
 		"collector.process.whitelist",