Support user-defined table stats collector

Summary:
1. Added a new option that support user-defined table stats collection.
2. Added a deleted key stats collector in `utilities`

Test Plan:
Added a unit test for newly added code.
Also ran make check to make sure other tests are not broken.

Reviewers: dhruba, haobo

Reviewed By: dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D13491

include/rocksdb/options.h

@@ -10,11 +10,13 @@
 #include <memory>
 #include <vector>
 #include <stdint.h>
-#include "rocksdb/slice.h"
-#include "rocksdb/statistics.h"
-#include "rocksdb/universal_compaction.h"
+
 #include "rocksdb/memtablerep.h"
+#include "rocksdb/slice.h"
 #include "rocksdb/slice_transform.h"
+#include "rocksdb/statistics.h"
+#include "rocksdb/table_stats.h"
+#include "rocksdb/universal_compaction.h"
 
 namespace rocksdb {
 
@@ -586,6 +588,12 @@ struct Options {
   // to data file.
   // Default: true
   bool purge_log_after_memtable_flush;
+
+  // This option allows user to to collect their own interested statistics of
+  // the tables.
+  // Default: emtpy vector -- no user-defined statistics collection will be
+  // performed.
+  std::vector<std::shared_ptr<TableStatsCollector>> table_stats_collectors;
 };
 
 //

include/rocksdb/table_stats.h

@@ -6,14 +6,14 @@
 #include <string>
 #include <unordered_map>
 
+#include "rocksdb/status.h"
+
 namespace rocksdb {
 
 // TableStats contains a bunch of read-only stats of its associated
 // table.
 struct TableStats {
  public:
-  // TODO(kailiu) we do not support user collected stats yet.
-  //
   // Other than basic table stats, each table may also have the user
   // collected stats.
   // The value of the user-collected stats are encoded as raw bytes --
@@ -43,4 +43,24 @@ struct TableStats {
   UserCollectedStats user_collected_stats;
 };
 
+// `TableStatsCollector` provides the mechanism for users to collect their own
+// interested stats. This class is essentially a collection of callback
+// functions that will be invoked during table building.
+class TableStatsCollector {
+ public:
+  virtual ~TableStatsCollector() { }
+  // Add() will be called when a new key/value pair is inserted into the table.
+  // @params key    the original key that is inserted into the table.
+  // @params value  the original value that is inserted into the table.
+  virtual Status Add(const Slice& key, const Slice& value) = 0;
+
+  // Finish() will be called when a table has already been built and is ready
+  // for writing the stats block.
+  // @params stats  User will add their collected statistics to `stats`.
+  virtual Status Finish(TableStats::UserCollectedStats* stats) = 0;
+
+  // The name of the stats collector can be used for debugging purpose.
+  virtual const char* Name() const = 0;
+};
+
 }  // namespace rocksdb