better documentation for tags

[alexism]

Kafka 0.8.2 refactored its metrics to use tags/scopes. See https://issues.apache.org/jira/browse/KAFKA-1481

In 0.4.0, `kafka-statsd-metrics21 introduced support for metric tags. Tags are an extension to the Statsd protocol introduced by Datadog, see http://docs.datadoghq.com/guides/dogstatsd/#tags.

By default, tags are added to the generated statsd metrics, but people using a Statsd server that does not support tags are confused.

The documentation should make these details more obvious and have clear explanation on how to disable tags. Examples of metrics generated when tags are off will also be useful.

[gdubicki]

+1

Only when I read this particular comment #9 (comment) I undestood how will your lib work with a standard, non-datadog statsd.

It woud be really nice if the docs were more clear on this subject then.