graphite: normalize metric name before sending

Better to be safe.
http://www.szuhanchang.com/2013/08/03/valid-characters-in-graphite-metric-name.html
http://docs.datadoghq.com/faq/#api-metric-names
https://github.com/daniellawrence/graphitesend/blob/master/src/graphitesend.py#L167