output perf_context in db_bench readrandom

Summary:
Add helper function to print perf context data in db_bench if enabled.
I didn't find any code that actually exports perf context data. Not sure
if I missed anything

Test Plan: ran db_bench

Reviewers: haobo, sdong, igor

Reviewed By: igor

CC: leveldb

Differential Revision: https://reviews.facebook.net/D16575

db/db_bench.cc

@@ -1994,7 +1994,12 @@ class Benchmark {
     char msg[100];
     snprintf(msg, sizeof(msg), "(%" PRIu64 " of %" PRIu64 " found)",
              found, reads_);
+
     thread->stats.AddMessage(msg);
+
+    if (FLAGS_perf_level > 0) {
+      thread->stats.AddMessage(perf_context.ToString());
+    }
   }
 
   void PrefixScanRandom(ThreadState* thread) {