Print elapsed time and number of operations completed (#9886)

Summary:
This is inspired by debugging a regression test that runs for ~0.05 seconds and the short
running time makes it prone to variance. While db_bench ran for ~60 seconds, 59.95 seconds
was spent opening 128 databases (and doing recovery). So it was harder to notice that the
benchmark only ran for 0.05 seconds.

Normally I add output to the end of the line to make life easier for existing tools that parse it
but in this case the output near the end of the line has two optional parts and one of the optional
parts adds an extra newline.

This is for https://github.com/facebook/rocksdb/issues/9856

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9886

Test Plan:
./db_bench --benchmarks=overwrite,readrandom --num=1000000 --threads=4

old output:
 DB path: [/tmp/rocksdbtest-2260/dbbench]
 overwrite    :      14.108 micros/op 283338 ops/sec;   31.3 MB/s
 DB path: [/tmp/rocksdbtest-2260/dbbench]
 readrandom   :       7.994 micros/op 496788 ops/sec;   55.0 MB/s (1000000 of 1000000 found)

new output:
 DB path: [/tmp/rocksdbtest-2260/dbbench]
 overwrite    :      14.117 micros/op 282862 ops/sec 14.141 seconds 4000000 operations;   31.3 MB/s
 DB path: [/tmp/rocksdbtest-2260/dbbench]
 readrandom   :       8.649 micros/op 458475 ops/sec 8.725 seconds 4000000 operations;   49.8 MB/s (981548 of 1000000 found)

Reviewed By: ajkr

Differential Revision: D36102269

Pulled By: mdcallag

fbshipit-source-id: 5cd8a9e11f5cbe2a46809571afd83335b6b0caa0

tools/db_bench_tool.cc

@@ -2265,25 +2265,23 @@ class Stats {
     if (done_ < 1) done_ = 1;
 
     std::string extra;
+    double elapsed = (finish_ - start_) * 1e-6;
     if (bytes_ > 0) {
       // Rate is computed on actual elapsed time, not the sum of per-thread
       // elapsed times.
-      double elapsed = (finish_ - start_) * 1e-6;
       char rate[100];
       snprintf(rate, sizeof(rate), "%6.1f MB/s",
                (bytes_ / 1048576.0) / elapsed);
       extra = rate;
     }
     AppendWithSpace(&extra, message_);
-    double elapsed = (finish_ - start_) * 1e-6;
     double throughput = (double)done_/elapsed;
 
-    fprintf(stdout, "%-12s : %11.3f micros/op %ld ops/sec;%s%s\n",
-            name.ToString().c_str(),
-            seconds_ * 1e6 / done_,
-            (long)throughput,
-            (extra.empty() ? "" : " "),
-            extra.c_str());
+    fprintf(stdout,
+            "%-12s : %11.3f micros/op %ld ops/sec %.3f seconds %" PRIu64
+            " operations;%s%s\n",
+            name.ToString().c_str(), seconds_ * 1e6 / done_, (long)throughput,
+            elapsed, done_, (extra.empty() ? "" : " "), extra.c_str());
     if (FLAGS_histogram) {
       for (auto it = hist_.begin(); it != hist_.end(); ++it) {
         fprintf(stdout, "Microseconds per %s:\n%s\n",