Supprt for SensuPluginMetricJSON

[lk3687051]

Hi dear:
Can we have a class which can gen metrics like SensuPluginMetricJSON and then use push_event

[barryorourke]

I've been planning to take a look at the metrics classes and bring them in line with the Ruby gem equivalent to this package.

We have a lot of changes going into the next release, so I'll take a look at it after we get that out.