Add histogram for DB_SEEK

Summary: as title

Test Plan: make release

Reviewers: sdong, yhchiang

Reviewed By: yhchiang

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D21717

db/db_iter.cc

@@ -552,6 +552,8 @@ void DBIter::FindParseableKey(ParsedInternalKey* ikey, Direction direction) {
 }
 
 void DBIter::Seek(const Slice& target) {
+  StopWatch sw(env_, statistics_, DB_SEEK);
+
   saved_key_.Clear();
   // now savved_key is used to store internal key.
   saved_key_.SetInternalKey(target, sequence_);

include/rocksdb/statistics.h

@@ -219,6 +219,7 @@ enum Histograms : uint32_t {
   HARD_RATE_LIMIT_DELAY_COUNT,
   SOFT_RATE_LIMIT_DELAY_COUNT,
   NUM_FILES_IN_SINGLE_COMPACTION,
+  DB_SEEK,
   HISTOGRAM_ENUM_MAX,
 };
 
@@ -241,6 +242,7 @@ const std::vector<std::pair<Histograms, std::string>> HistogramsNameMap = {
   { HARD_RATE_LIMIT_DELAY_COUNT, "rocksdb.hard.rate.limit.delay.count"},
   { SOFT_RATE_LIMIT_DELAY_COUNT, "rocksdb.soft.rate.limit.delay.count"},
   { NUM_FILES_IN_SINGLE_COMPACTION, "rocksdb.numfiles.in.singlecompaction" },
+  { DB_SEEK, "rocksdb.db.seek.micros" },
 };
 
 struct HistogramData {