Add event remapping functionality

[mike-sol]

The plugin works fine, but unfortunately the way it structures the events that it creates is less than ideal.

In my particular case, I tried to use the Netapp Harvester (which gets metrics from a Netapp storage system and outputs plaintext Graphite) with this input plugin so as to not have to spin up an actual Graphite server.

The result was a bunch of events with field names like:
netapp.capacity.Cluster-HQ.Cluster-HQ.node.Cluster-HQ-01.aggr.cluster_hq_01_aggr1.snapshot_reserve_avail

Elasticsearch quickly began to throw errors when the number of fields in the (brand new, empty) index hit 1000. It occurs to me that any reasonable Graphite deployment using a metrics collector would similarly hit those kinds of numbers.

What I would like to do is to have some options to re-map the name.

Option 1 could be as simple as just having a field named 'metric' containing the metric name, and a separate field called value with the value. May be some issue with conflicts for non-numeric values, might have to go with a text or similarly named field for those instead of value. If nothing else, this would make the plugin perfectly useful for me.

Option 2 would be more complex, you'd split on dots, and then with the resulting data create a hierarchy, so you'd map from:

netapp.capacity.Cluster-HQ.Cluster-HQ.node.Cluster-HQ-01.aggr.cluster_hq_01_aggr1.snapshot_reserve_avail = 100

to

metric.1 = netapp
metric.2 = capacity
metric.3 = Cluster-HQ
....
value = 100

This option would enable the canny Elasticsearcher to effectively be able to query the hierarchy the same way as a Graphiter would with wildcards.