Fix num files in single compaction for universal compaction (#9168)

Summary:
https://github.com/facebook/rocksdb/issues/9026 fixed histogram NUM_FILES_IN_SINGLE_COMPACTION for level compaction, but missed fix for universal compaction.

This PR fixed NUM_FILES_IN_SINGLE_COMPACTION for universal compaction.

Quote from https://github.com/facebook/rocksdb/issues/9026:
> 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.

Thanks for ajkr pointed this missed fix!

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9168

Reviewed By: akankshamahajan15

Differential Revision: D32434494

Pulled By: ajkr

fbshipit-source-id: 93ea092af4afbd8dce67898ffb350cf26b065ed2

HISTORY.md

@@ -52,6 +52,9 @@
 ### Performance Improvements
 * Released some memory related to filter construction earlier in `BlockBasedTableBuilder` for `FullFilter` and `PartitionedFilter` case (#9070)
 
+### 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/compaction/compaction_picker_universal.cc

@@ -498,8 +498,11 @@ Compaction* UniversalCompactionBuilder::PickCompaction() {
   }
 #endif
   // update statistics
-  RecordInHistogram(ioptions_.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(ioptions_.stats, NUM_FILES_IN_SINGLE_COMPACTION, num_files);
 
   picker_->RegisterCompaction(c);
   vstorage_->ComputeCompactionScore(ioptions_, mutable_cf_options_);