Deserialize custom Statistics object in db_bench

Summary:
Added -statistics_string to deserialize a Statistics object using the factory functions registered by applications.
Closes https://github.com/facebook/rocksdb/pull/1812

Differential Revision: D4469811

Pulled By: ajkr

fbshipit-source-id: 2d80862

tools/db_bench_tool.cc

@@ -459,6 +459,7 @@ DEFINE_bool(verify_checksum, false, "Verify checksum for every block read"
             " from storage");
 
 DEFINE_bool(statistics, false, "Database statistics");
+DEFINE_string(statistics_string, "", "Serialized statistics string");
 static class std::shared_ptr<rocksdb::Statistics> dbstats;
 
 DEFINE_int64(writes, -1, "Number of write operations to do. If negative, do"
@@ -4878,6 +4879,24 @@ int db_bench_tool(int argc, char** argv) {
   ParseCommandLineFlags(&argc, &argv, true);
 
   FLAGS_compaction_style_e = (rocksdb::CompactionStyle) FLAGS_compaction_style;
+#ifndef ROCKSDB_LITE
+  if (FLAGS_statistics && !FLAGS_statistics_string.empty()) {
+    fprintf(stderr,
+            "Cannot provide both --statistics and --statistics_string.\n");
+    exit(1);
+  }
+  if (!FLAGS_statistics_string.empty()) {
+    std::unique_ptr<Statistics> custom_stats_guard;
+    dbstats.reset(NewCustomObject<Statistics>(FLAGS_statistics_string,
+                                              &custom_stats_guard));
+    custom_stats_guard.release();
+    if (dbstats == nullptr) {
+      fprintf(stderr, "No Statistics registered matching string: %s\n",
+              FLAGS_statistics_string.c_str());
+      exit(1);
+    }
+  }
+#endif  // ROCKSDB_LITE
   if (FLAGS_statistics) {
     dbstats = rocksdb::CreateDBStatistics();
   }