Adds pre-aggregation for the metrics filter

This allows users to add a count to their events if they are already
pre-aggregated. This is useful when you have a lot of events that
Logstash could not process, coming from multiple places, and you want to
aggregate them without sending an event for each of them.

For example, in our situation, without this we would need to send around 120k events per second.

This is optimal for situations where you do not want per-event details but care more about the rate/speed of a huge amount of events.