Emit DataDog style statsd tags.

This commit allows metrics names to have a trailing number of tuples
which will be converted into a list of tags in the DataDog style. See
http://docs.datadoghq.com/guides/dogstatsd/ for more details.

This change is backward compatible with stock statsd.

Signed-off-by: Brian L. Troutwine <brian.troutwine@adroll.com>