Add Sum in Histogram librato measurement

Gauge sum is required in librato API when count is present :

> If count is set, then sum must also be set in order to calculate an average value for the recorded metric measurement. 

from [Gauge Specific Parameters](http://dev.librato.com/v1/post/metrics)

Without this attribute, the request is refused : 

    2014/10/12 16:59:54 ERROR sending metrics to librato Unable to post to Librato: 400 400 Bad Request {"errors":{"params":{"sum":["is required"],"name":"buffer.SizeHistory.hist"}}}