Misc heka work

This is a bit of a grab bag, feel free to question/reject/demand reworking of any part of it. It accumulated over the course of a couple days looking around heka, heka-py and our metrics reporting.

It's got 3 main parts: adding gauge support to the library and heka config, adding some new gauges that seemed like they might be helpful to keep an eye on (without drowning in new metrics), and making a couple tasks that are obvious metrics-gathering places for future miscellaneous gauges.

The system-level psutil stuff in particular I don't know whether we want to gather. I realize AWS has plenty of monitoring machinery for this already, but it seemed to me that it might be nice to have redundant metrics flowing through our own heka pipeline, for plotting / correlating adjacent to application-level metrics, and even just for ensuring that they get noticed if they go sideways.