[filter/metrics] Fix flushing in metrics filter

As discovered in #1805, metrics filter is not correctly flushing events

The current spec was calling flush manually not relying on the pipeline periodic_flusher threads, 
here is a proposal test case using the full agent to cover this case + fix to metrics filter based on the changed done to other flushing filters (multiline, spool) in #1545 
Based on this, I've commented - https://github.com/elasticsearch/logstash/pull/1545/files#r18324254

I don't think this setting should exist for metrics filter. Flushing at this time is *not* optional, so allowing users to set it to false seems weird.