The site metrics API should be documented

[tlevine]

The site metrics API (/api/site_metrics.json) should be documented, unless it's going to change soon.

I can take this one; I just have to copy things from here.

[jkew]

There's a few changes coming in soon; specifically we will be differentiating between datasets and snapshots of datasets. The API will remain generally the same; but we are planning a revamp of the analytics page which consumes it; migration to D3; along with a more sensible layout.

[chrismetcalf]

When we go through the next rewrite of the metrics API, I'll push for making it a public API when the metrics themselves are public. Marking this as a future issue to be tracked.