metrics output should allow backfill

[talevy]

The metrics {} output should probably allow backfilling on logs based upon timestamp from the log for eventual loading into tools like Graphite. In this way users won't have to manually connect to Carbon or similar tools in an attempt to reload their metrics, simply point logstash's configuration at the old logs.

migrated from: https://logstash.jira.com/browse/LOGSTASH-752

[abesto]

+1, this would be very useful for us