Add Sum in Histogram librato measurement

[albanpeignier]

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

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"}}}

[mihasya]

Puzzling. I just added a histogram to a service a few days ago, have data in the dashboard, and no errors in the logs. However, the error appears to be very clear and corresponds to the docs.

Would you mind getting client.go to print out the exact payload being posted so I have something with which to ping the librato folks? Thanks!

[mihasya]

Sorry about the delay, confirming that this was broken by this commit d4f1d62 had to do a bit of git bisect to get to the bottom of it.

[mihasya]

Also requires a fix to Timer so let me merge this and push that one next.

[rcrowley]

Herp derp, thanks @albanpeignier and @mihasya.