Deprecate Hostname usage in DogStatsdSink

[smoya]

hostname is only being used for pulling it out when matched as metric prefix:
https://github.com/armon/go-metrics/blob/master/datadog/dogstatsd.go#L63-L82

This is not only not documented but also a very unexpected behaviour that has caused us some problems in the past (where are my metrics?) -- See netlify/netlify-commons#140

I can submit a PR if needed.