Counters for bytes written and read.

Summary:
* Counters for bytes read and write.
as a part of this diff, I want to=>
* Measure compaction times. @dhruba can you point which function, should
* I time to get Compaction-times. Was looking at CompactRange.

Test Plan: db_test

Reviewers: dhruba, emayanke

CC: leveldb

Differential Revision: https://reviews.facebook.net/D8763

include/leveldb/statistics.h

@@ -33,9 +33,13 @@ enum Tickers {
   NUMBER_KEYS_WRITTEN = 6,
   // Number of Keys read,
   NUMBER_KEYS_READ = 7,
-  TICKER_ENUM_MAX = 8,
+  // Bytes written / read
+  BYTES_WRITTEN = 8,
+  BYTES_READ = 9,
+  TICKER_ENUM_MAX = 10,
 };
 
+
 /**
  * Keep adding histogram's here.
  * Any histogram whould have value less than HISTOGRAM_ENUM_MAX