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

db/db_impl.cc

@@ -1855,6 +1855,7 @@ Status DBImpl::Get(const ReadOptions& options,
   imm.UnrefAll();
   current->Unref();
   RecordTick(options_.statistics, NUMBER_KEYS_READ);
+  RecordTick(options_.statistics, BYTES_READ, value->size());
   return s;
 }
 
@@ -1917,7 +1918,9 @@ Status DBImpl::Write(const WriteOptions& options, WriteBatch* my_batch) {
     last_sequence += my_batch_count;
     // Record statistics
     RecordTick(options_.statistics, NUMBER_KEYS_WRITTEN, my_batch_count);
-
+    RecordTick(options_.statistics,
+               BYTES_WRITTEN,
+               WriteBatchInternal::ByteSize(updates));
     // Add to log and apply to memtable.  We can release the lock
     // during this phase since &w is currently responsible for logging
     // and protects against concurrent loggers and concurrent writes

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