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/cache.go

@@ -8,10 +8,6 @@ import (
 	"github.com/prometheus/client_golang/prometheus"
 )
 
-func init() {
-	registerCollector("cache", newCacheCollector, "Cache")
-}
-
 // A CacheCollector is a Prometheus collector for Perflib Cache metrics
 type CacheCollector struct {
 	AsyncCopyReadsTotal         *prometheus.Desc