Migrate to new client_golang.

This should only be merged once the new client_golang is merged to its
master branch.
counter -> total.
Merge pull request #2 from prometheus/migration/new-client-golang

Migrate to new client_golang.
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.