Simplify the log format for Dry Run

- Omit the severity initial and progname
- Show the elapsed time in HH:MM:SS instead of full date-time

app/concerns/dry_runnable.rb

@@ -5,7 +5,12 @@ module DryRunnable
     @dry_run = true
 
     log = StringIO.new
-    @dry_run_logger = Logger.new(log)
+    @started_at = Time.zone.now
+    @dry_run_logger = Logger.new(log).tap { |logger|
+      logger.formatter = proc { |severity, datetime, progname, message|
+        "[#{elapsed_time(datetime)}] #{severity} -- #{progname}: #{message}\n"
+      }
+    }
     @dry_run_results = {
       events: [],
     }
@@ -13,13 +18,17 @@ module DryRunnable
     begin
       raise "#{short_type} does not support dry-run" unless can_dry_run?
       readonly!
+      @started_at = Time.zone.now
+      @dry_run_logger.info('Dry Run started')
       if event
         raise "This agent cannot receive an event!" unless can_receive_events?
         receive([event])
       else
         check
       end
+      @dry_run_logger.info('Dry Run finished')
     rescue => e
+      @dry_run_logger.info('Dry Run failed')
       error "Exception during dry-run. #{e.message}: #{e.backtrace.join("\n")}"
     end
 
@@ -35,6 +44,12 @@ module DryRunnable
     !!@dry_run
   end
 
+  def elapsed_time(datetime)
+    '%02d:%02d:%02d' % 2.times.inject([datetime - @started_at]) { |(x, *xs)|
+      [*x.divmod(60), *xs]
+    }
+  end
+
   included do
     prepend Wrapper
   end