db_bench: fix "micros/op" reporting (#4949)

Summary:
4985a9f73b (diff-e5276985b26a0551957144f4420a594bR511)
changes the meaning of latency reporting from running time per query, to elapse_time / #ops, without providing a reason why.
Considering that this is a counter-intuitive reporting, Reverting the change.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4949

Differential Revision: D13964684

Pulled By: siying

fbshipit-source-id: d6304d3d4b5a802daa292302623c7dbca9a680bc

tools/db_bench_tool.cc

@@ -1831,7 +1831,7 @@ class Stats {
 
     fprintf(stdout, "%-12s : %11.3f micros/op %ld ops/sec;%s%s\n",
             name.ToString().c_str(),
-            elapsed * 1e6 / done_,
+            seconds_ * 1e6 / done_,
             (long)throughput,
             (extra.empty() ? "" : " "),
             extra.c_str());