add missing counters in readonly mode (#4260)

Summary:
User reported (https://github.com/facebook/rocksdb/issues/4168) that when opening RocksDB in read-only mode, some statistics are not correctly reported. After some investigation, we believe the following counters are indeed not reported during Get() call in a read-only DB:
rocksdb.memtable.hit
rocksdb.memtable.miss
rocksdb.number.keys.read
rocksdb.bytes.read
As well as histogram rocksdb.bytes.per.read
and perf context get_read_bytes
This PR will add the necessary counter reporting logic in the Get() call path
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4260

Differential Revision: D9476431

Pulled By: miasantreble

fbshipit-source-id: 7ab409d4e59df05d09ae8b69fe75554e5aa240d6

db/db_impl_readonly.cc

@@ -31,6 +31,8 @@ Status DBImplReadOnly::Get(const ReadOptions& read_options,
                            ColumnFamilyHandle* column_family, const Slice& key,
                            PinnableSlice* pinnable_val) {
   assert(pinnable_val != nullptr);
+  // TODO: stopwatch DB_GET needed?, perf timer needed?
+  PERF_TIMER_GUARD(get_snapshot_time);
   Status s;
   SequenceNumber snapshot = versions_->LastSequence();
   auto cfh = reinterpret_cast<ColumnFamilyHandleImpl*>(column_family);
@@ -45,14 +47,22 @@ Status DBImplReadOnly::Get(const ReadOptions& read_options,
   MergeContext merge_context;
   RangeDelAggregator range_del_agg(cfd->internal_comparator(), snapshot);
   LookupKey lkey(key, snapshot);
+  PERF_TIMER_STOP(get_snapshot_time);
   if (super_version->mem->Get(lkey, pinnable_val->GetSelf(), &s, &merge_context,
                               &range_del_agg, read_options)) {
     pinnable_val->PinSelf();
+    RecordTick(stats_, MEMTABLE_HIT);
   } else {
     PERF_TIMER_GUARD(get_from_output_files_time);
     super_version->current->Get(read_options, lkey, pinnable_val, &s,
                                 &merge_context, &range_del_agg);
+    RecordTick(stats_, MEMTABLE_MISS);
   }
+  RecordTick(stats_, NUMBER_KEYS_READ);
+  size_t size = pinnable_val->size();
+  RecordTick(stats_, BYTES_READ, size);
+  MeasureTime(stats_, BYTES_PER_READ, size);
+  PERF_COUNTER_ADD(get_read_bytes, size);
   return s;
 }
 

db/perf_context_test.cc

@@ -469,7 +469,7 @@ void ProfileQueries(bool enabled_time = false) {
     ASSERT_GT(hist_num_memtable_checked.Average(), 0);
     // In read-only mode Get(), no super version operation is needed
     ASSERT_EQ(hist_get_post_process.Average(), 0);
-    ASSERT_EQ(hist_get_snapshot.Average(), 0);
+    ASSERT_GT(hist_get_snapshot.Average(), 0);
 
     ASSERT_GT(hist_mget.Average(), 0);
     ASSERT_GT(hist_mget_snapshot.Average(), 0);