Add ability to use fields as metrics, typically set by the kv filter

Add ability to use fields as metrics, typically set by the kv filter like:

filter { kv { } }
output { statsd { fields_are_metrics => true } }

The intention is to allow developers to be able to set any event occurrence in log files without any need of server side logstash configuration for the metric. They can log lines like:

company_metric: foo.bar_count=3, baz.qux_increment=1

These will be picked up by logstash and sent selectively to the kv filter by (grok or tags) which will then send to statsd