Add compaction time to log output

Summary:
Although compaction time is recorded in the statistics,
it is helpful to include this value in the log output corresponding
to the end of compaction.

Test Plan: make all && make check

Reviewers: yhchiang, sdong, igor, noetzli, MarkCallaghan

Reviewed By: MarkCallaghan

Subscribers: dhruba

Differential Revision: https://reviews.facebook.net/D47007

db/compaction_job.cc

@@ -542,8 +542,9 @@ Status CompactionJob::Install(const MutableCFOptions& mutable_cf_options,
   UpdateCompactionJobStats(stats);
 
   auto stream = event_logger_->LogToBuffer(log_buffer_);
-  stream << "job" << job_id_ << "event"
+  stream << "job" << job_id_
-         << "compaction_finished"
+         << "event" << "compaction_finished"
+         << "compaction_time_micros" << compaction_stats_.micros
          << "output_level" << compact_->compaction->output_level()
          << "num_output_files" << compact_->NumOutputFiles()
          << "total_output_size" << compact_->total_bytes