Add a stats counter for DB_WRITE back which was mistakenly removed.

Summary: Add a stats counter for DB_WRITE back which was mistakenly removed.

Test Plan: augment GroupCommitTest

Reviewers: sdong

Reviewed By: sdong

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D39399

db/db_impl.cc

@@ -3223,6 +3223,8 @@ Status DBImpl::WriteImpl(const WriteOptions& write_options,
     RecordTick(stats_, WRITE_WITH_WAL);
   }
 
+  StopWatch write_sw(env_, db_options_.statistics.get(), DB_WRITE);
+
   WriteContext context;
   mutex_.Lock();
 

db/db_test.cc

@@ -8591,6 +8591,9 @@ TEST_F(DBTest, GroupCommitTest) {
     ASSERT_TRUE(!itr->Valid());
     delete itr;
 
+    HistogramData hist_data = {0};
+    options.statistics->histogramData(DB_WRITE, &hist_data);
+    ASSERT_GT(hist_data.average, 0.0);
   } while (ChangeOptions(kSkipNoSeekToLast));
 }