Use CounterVisitor for object cache

Summary: As title. We want to kill the isRate check in ServiceDataExporter

Reviewed By: jiayuebao

Differential Revision: D41112361

fbshipit-source-id: fb5c80631fe970d2d05de4ef860db5f5432c1fa7

Author: haowu14

cachelib/allocator/Cache.h

@@ -26,6 +26,7 @@
 #include "cachelib/allocator/ICompactCache.h"
 #include "cachelib/allocator/memory/MemoryAllocator.h"
 #include "cachelib/common/Hash.h"
+#include "cachelib/common/Utils.h"
 
 namespace facebook {
 namespace cachelib {
@@ -194,8 +195,7 @@ class CacheBase {
   virtual const ICompactCache& getCompactCache(PoolId pid) const = 0;
 
   // return object cache stats
-  virtual void getObjectCacheCounters(
-      std::function<void(folly::StringPiece, uint64_t)>) const {}
+  virtual void getObjectCacheCounters(const util::CounterVisitor&) const {}
 
  protected:
   // move bytes from one pool to another. The source pool should be at least

cachelib/experimental/objcache2/ObjectCache-inl.h

@@ -313,7 +313,7 @@ void ObjectCache<AllocatorT>::remove(folly::StringPiece key) {
 
 template <typename AllocatorT>
 void ObjectCache<AllocatorT>::getObjectCacheCounters(
-    std::function<void(folly::StringPiece, uint64_t)> visitor) const {
+    const util::CounterVisitor& visitor) const {
   visitor("objcache.lookups", lookups_.get());
   visitor("objcache.lookups.l1_hits", succL1Lookups_.get());
   visitor("objcache.inserts", inserts_.get());

cachelib/experimental/objcache2/ObjectCache.h

@@ -206,7 +206,7 @@ class ObjectCache : public ObjectCacheBase<AllocatorT> {
   // @param visitor   callback that will be invoked with
   //                  {stat-name, value} for each stat
   void getObjectCacheCounters(
-      std::function<void(folly::StringPiece, uint64_t)> visitor) const override;
+      const util::CounterVisitor& visitor) const override;
 
   // Return the number of objects in cache
   uint64_t getNumEntries() const {