[Rocksdb] measure table open io in a histogram

Summary: as title

Test Plan: db_bench --statistics=1 check for statistic.

Reviewers: dhruba, haobo

Reviewed By: dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D11109

include/leveldb/statistics.h

@@ -73,8 +73,10 @@ enum Histograms {
   COMPACTION_OUTFILE_SYNC_MICROS = 4,
   WAL_FILE_SYNC_MICROS = 5,
   MANIFEST_FILE_SYNC_MICROS = 6,
-  DB_MULTIGET = 7,
-  HISTOGRAM_ENUM_MAX = 8
+  // TIME SPENT IN IO DURING TABLE OPEN
+  TABLE_OPEN_IO_MICROS = 7,
+  DB_MULTIGET = 8,
+  HISTOGRAM_ENUM_MAX = 9
 };
 
 struct HistogramData {