fix: export prometheus metrics for cache GC triggers (#9815)

Bonus change to use channel to serialize triggers,
instead of using atomic variables. More efficient
mechanism for synchronization.

Co-authored-by: Nitish Tiwari <nitish@minio.io>

cmd/disk-cache-utils.go

@@ -439,6 +439,14 @@ func (f *fileScorer) fileObjInfos() []ObjectInfo {
 	return res
 }
 
+func (f *fileScorer) purgeFunc(p func(qfile queuedFile)) {
+	e := f.queue.Front()
+	for e != nil {
+		p(e.Value.(queuedFile))
+		e = e.Next()
+	}
+}
+
 // fileNames returns all queued file names.
 func (f *fileScorer) fileNames() []string {
 	res := make([]string, 0, f.queue.Len())