Support datadog extensions to statsd datagrams

Datadog's format extensions allow attaching labels and tags to statsd
metrics, and are documented at
http://docs.datadoghq.com/guides/dogstatsd/#datagram-format

Additionally changes compare method in tests to reflect.DeepEqual, as
the tag maps don't sort deterministically, and treats an invalid sample
value as 1 instead of throwing out the sample.