fix histogram NUM_FILES_IN_SINGLE_COMPACTION (#9026)

Summary:
currently histogram `NUM_FILES_IN_SINGLE_COMPACTION` just counted files in first level of compaction input, this fix counts files in all levels of compaction input.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9026

Reviewed By: ajkr

Differential Revision: D31668241

Pulled By: jay-zhuang

fbshipit-source-id: c02f6c4a5df9fbf0b7510036594811152e8738af

HISTORY.md

@@ -8,6 +8,9 @@
 * Fixed a bug in CompactionIterator when write-prepared transaction is used. A released earliest write conflict snapshot may cause assertion failure in dbg mode and unexpected key in opt mode.
 * Fix ticker WRITE_WITH_WAL("rocksdb.write.wal"), this bug is caused by a bad extra `RecordTick(stats_, WRITE_WITH_WAL)` (at 2 place), this fix remove the extra `RecordTick`s and fix the corresponding test case.
 
+### Behavior Changes
+* `NUM_FILES_IN_SINGLE_COMPACTION` was only counting the first input level files, now it's including all input files.
+
 ## 6.26.0 (2021-10-20)
 ### Bug Fixes
 * Fixes a bug in directed IO mode when calling MultiGet() for blobs in the same blob file. The bug is caused by not sorting the blob read requests by file offsets.

db/db_impl/db_impl_compaction_flush.cc

@@ -3076,8 +3076,12 @@ Status DBImpl::BackgroundCompaction(bool* made_progress,
           status = Status::CompactionTooLarge();
         } else {
           // update statistics
-          RecordInHistogram(stats_, NUM_FILES_IN_SINGLE_COMPACTION,
-                            c->inputs(0)->size());
+          size_t num_files = 0;
+          for (auto& each_level : *c->inputs()) {
+            num_files += each_level.files.size();
+          }
+          RecordInHistogram(stats_, NUM_FILES_IN_SINGLE_COMPACTION, num_files);
+
           // There are three things that can change compaction score:
           // 1) When flush or compaction finish. This case is covered by
           // InstallSuperVersionAndScheduleWork