Add a statistic to count the number of calls to GetUpdatesSince

Summary: This is useful to keep track of refreshes in transaction log iterator

Test Plan: make; db_stress --statistics=1 shows it

Reviewers: dhruba, haobo

Reviewed By: dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D13281

db/db_impl.cc

@@ -1092,6 +1092,7 @@ SequenceNumber DBImpl::GetLatestSequenceNumber() {
 Status DBImpl::GetUpdatesSince(SequenceNumber seq,
                                unique_ptr<TransactionLogIterator>* iter) {
 
+  RecordTick(options_.statistics, GET_UPDATES_SINCE_CALLS);
   if (seq > last_flushed_sequence_) {
     return Status::IOError("Requested sequence not yet written in the db");
   }

include/rocksdb/statistics.h

@@ -76,6 +76,10 @@ enum Tickers {
   // over large number of keys with same userkey.
   NUMBER_OF_RESEEKS_IN_ITERATION,
 
+  // Record the number of calls to GetUpadtesSince. Useful to keep track of
+  // transaction log iterator refreshes
+  GET_UPDATES_SINCE_CALLS,
+
   TICKER_ENUM_MAX
 };
 
@@ -108,7 +112,8 @@ const std::vector<std::pair<Tickers, std::string>> TickersNameMap = {
   { SEQUENCE_NUMBER, "rocksdb.sequence.number" },
   { BLOOM_FILTER_PREFIX_CHECKED, "rocksdb.bloom.filter.prefix.checked" },
   { BLOOM_FILTER_PREFIX_USEFUL, "rocksdb.bloom.filter.prefix.useful" },
-  { NUMBER_OF_RESEEKS_IN_ITERATION, "rocksdb.number.reseeks.iteration" }
+  { NUMBER_OF_RESEEKS_IN_ITERATION, "rocksdb.number.reseeks.iteration" },
+  { GET_UPDATES_SINCE_CALLS, "rocksdb.getupdatessince.calls" }
 };
 
 /**