Add an OpenTracing span for each rule (#4027)
Add an OpenTracing span for each rule (#4027)

* Add an OpenTracing span for each rule

So that tags and child spans can be traced back to the rule that they
refer to.

Gouthamve/prometheus,alileza/prometheus,GoogleCloudPlatform/prometheus,GoogleCloudPlatform/prometheus,Gouthamve/prometheus,prometheus/prometheus,alileza/prometheus,amorken/prometheus,GoogleCloudPlatform/prometheus,keegancsmith/prometheus,prometheus/prometheus,kangwoo/prometheus,keegancsmith/prometheus,keegancsmith/prometheus,Gouthamve/prometheus,prometheus/prometheus,GoogleCloudPlatform/prometheus,Gouthamve/prometheus,amorken/prometheus,alileza/prometheus,kangwoo/prometheus,amorken/prometheus,prometheus/prometheus,keegancsmith/prometheus,alileza/prometheus,keegancsmith/prometheus,prometheus/prometheus,kangwoo/prometheus,amorken/prometheus,GoogleCloudPlatform/prometheus,kangwoo/prometheus,alileza/prometheus