skip metriks for event that are older than defined threshold

If logstash was was down/restarted , then there will be lots of events with too old timestamp
it will skew metrics that are relevant only for current short time window.
So events that are older than @past_seconds, will not be used my metriks.

<pre>metrics {
    tags         => [ "apache-accesslog" ]
    meter        => [ "http.%{appenv}.status.%{status}" ]
    add_tag      => [ "apache-metrics" ]
    past_seconds => 10
  }</pre>