Update "num_data_read" stat in RetrieveMultipleBlocks (#7770)

Summary:
RetrieveMultipleBlocks which is used by MultiGet to read data blocks is not updating num_data_read stat in
GetContextStats.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7770

Test Plan: make check -j64

Reviewed By: anand1976

Differential Revision: D25538982

Pulled By: akankshamahajan15

fbshipit-source-id: e3daedb035b1be8ab6af6f115cb3793ccc7b1ec6

db/db_basic_test.cc

@@ -1961,7 +1961,7 @@ TEST_F(DBBasicTest, MultiGetStats) {
   ASSERT_GT(hist_sst.max, 0);
 
   // Minimun number of blocks read in a level.
-  ASSERT_EQ(hist_data_blocks.min, 0);
+  ASSERT_EQ(hist_data_blocks.min, 3);
   ASSERT_GT(hist_index_and_filter_blocks.min, 0);
   // Minimun number of sst files read in a level.
   ASSERT_GT(hist_sst.max, 0);

table/block_based/block_based_table_reader.cc

@@ -1768,6 +1768,9 @@ void BlockBasedTable::RetrieveMultipleBlocks(
     size_t& req_idx = req_idx_for_block[valid_batch_idx];
     size_t& req_offset = req_offset_for_block[valid_batch_idx];
     valid_batch_idx++;
+    if (mget_iter->get_context) {
+      ++(mget_iter->get_context->get_context_stats_.num_data_read);
+    }
     FSReadRequest& req = read_reqs[req_idx];
     Status s = req.status;
     if (s.ok()) {