[rocksdb][refactor] statistic printing code to one place

Summary: $title

Test Plan: db_bench --statistics=1

Reviewers: haobo

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D11373

include/leveldb/statistics.h

@@ -186,7 +186,8 @@ class Statistics {
   virtual void measureTime(Histograms histogramType, uint64_t time) = 0;
 
   virtual void histogramData(Histograms type, HistogramData * const data) = 0;
-
+  // String representation of the statistic object.
+  std::string ToString();
 };
 
 // Create a concrete DBStatistics object