Counter for "every" Setting is not threadsafe

[original-brownbear]

https://github.com/logstash-plugins/logstash-filter-sleep/blob/master/lib/logstash/filters/sleep.rb#L102

isn't thread safe. If I run multiple workers, the "every" setting will not work correctly 100% of the time. I think this is what's causing elastic/logstash#8073