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, corrects an inversion in
displaying test failure cases, and treats an invalid sample value as 1
instead of throwing out the sample.
Most people won't know about the Datadog extensions, so let's put a comment above this to explain what these labels mean (ideally with link to the Datadog docs).