Improve ruler tracing

[bboreham]

Using weaveworks/common/instrument in Evaluate() gives us a tracing span which ties together all the sub-operations for that rule group.

It would be better to have a span per rule, but that is inside Prometheus.

I also changed the histogram buckets for eval duration to go up to 25 seconds, and drop the low end below 100ms.

[jml]

Curious as to how this interacts with #719

[cboggs]

At a glance it seems like it should play nicely with #719, at least functionally. Looks like we're just threading UserID through the evaluation cycle to enhance traces, which should mesh reasonably well with the rules-by-file logic.

I'll merge the two locally to see how it turns out and report back shortly.

[cboggs]

Some conflicts on mutually modified lines, but I don't see anything functionally breaking.
A bit of the conflict will go away when the HistogramVec commit from #719 is reverted.
If / when this PR is merged, we can resolve the conflicts pretty easily on #719 and call it good.