heal: Add MRF metrics to background heal API response (#12398)

This commit gathers MRF metrics from 
all nodes in a cluster and return it to the caller. This will show information about the 
number of objects in the MRF queues 
waiting to be healed.

cmd/globals.go

@@ -293,6 +293,8 @@ var (
 	globalBackgroundHealRoutine *healRoutine
 	globalBackgroundHealState   *allHealState
 
+	globalMRFState *mrfState
+
 	// If writes to FS backend should be O_SYNC.
 	globalFSOSync bool