Adding the increment for a counter for a number of WAL syncs

Summary: This will unblock the corresponding change in MyRocks

Test Plan: ran rocksdb.write_sync test

Reviewers: sdong, kolmike

Subscribers: dhruba

Differential Revision: https://reviews.facebook.net/D46911

db/db_impl.cc

@@ -2002,6 +2002,7 @@ Status DBImpl::SyncWAL() {
     need_log_dir_sync = !log_dir_synced_;
   }
 
+  RecordTick(stats_, WAL_FILE_SYNCED);
   Status status;
   for (log::Writer* log : logs_to_sync) {
     status = log->file()->SyncWithoutFlush(db_options_.use_fsync);