According to #33449 and #33468, cpu_time and allocations are 0 when
JRuby is used.
```ruby
$ ruby -v
jruby 9.2.1.0-SNAPSHOT (2.5.0) 2018-07-27 13b2df5 Java HotSpot(TM) 64-Bit Server VM 25.181-b13 on 1.8.0_181-b13 [linux-x86_64]
$ bundle exec ruby -w -Itest test/log_subscriber_test.rb -n test_event_attributes
Run options: -n test_event_attributes --seed 6231
F
Failure:
SyncLogSubscriberTest#test_event_attributes [test/log_subscriber_test.rb:84]:
Expected 0 to be > 0.
rails test test/log_subscriber_test.rb:78
Finished in 0.018983s, 52.6791 runs/s, 105.3582 assertions/s.
1 runs, 2 assertions, 1 failures, 0 errors, 0 skips
```
* Use process clock instead of Time.now
This fixes any issues with the system clock changing and also eliminates
2 object allocations per event.
* Add start! and finish! methods to the event object so we can record
more information
* Adds cpu time, idle time, and allocation count for a particular event.
Co-authored-by: Aaron Patterson <aaron.patterson@gmail.com>
