Statsd metrics not isolated per plugin

[kostasb]

Running 2 statsd input plugins:

 [[inputs.statsd]]
   name_prefix = "gauges_"
   service_address = ":8125"
   metric_separator = "_"
   templates = ["measurement.atag"]
   allowed_pending_messages = 10000

 [[inputs.statsd]]
   name_prefix = "counters_"
   service_address = ":8126"
   metric_separator = "_"
   templates = ["measurement.atag"]
   allowed_pending_messages = 10000

Submit a single metric to one of the inputs, e.g. on port 8125:

echo "a.b:100|c" | nc -u localhost 8125

This results in two output metrics, both prefixes applied.

counters_a,atag=b,host=kubuntu,metric_type=counter value=100i 1477910760000000000
gauges_a,atag=b,host=kubuntu,metric_type=counter value=100i 1477910760000000000

Tested on the nightly build.