Small refactor of gc/scheduler to remove import of metadata

Replace metadata.GCStats with an interface for exposing elapsed time

Signed-off-by: Daniel Nephin <dnephin@gmail.com>

gc/gc.go

@@ -8,6 +8,7 @@ package gc
 import (
 	"context"
 	"sync"
+	"time"
 )
 
 // ResourceType represents type of resource at a node
@@ -21,6 +22,11 @@ type Node struct {
 	Key       string
 }
 
+// Stats about a garbage collection run
+type Stats interface {
+	Elapsed() time.Duration
+}
+
 // Tricolor implements basic, single-thread tri-color GC. Given the roots, the
 // complete set and a refs function, this function returns a map of all
 // reachable objects.