elasticsearch: support for grouping stats for logstash indices

ES keeps lots of stats per index, which are gathered by this ES collector if the 'stats' config contains 'indices'.

If you use Logstash to feed Elasticsearch, this means new indices every day.

Our ES cluster has nearly 1000 indexes (lots of different logtypes X retention). Every time the ES collector runs it forwards ~15.000 metrics to the configured handler(s).

I do not care about stats for every index (e.g. logstash-name-2014.01.03). I do care about stats per index group (e.g. logstash-name).

This PR enabled you to do just that.