Document Consul service ID label.
Document Consul service ID label.

Fixes https://github.com/prometheus/docs/issues/180

vic3lord/docs,vic3lord/docs,pdbogen/docs,Cepave/openfalcon.io,prometheus/docs,pdbogen/docs,open-falcon/open-falcon.github.io,ddfreyne/prometheus-docs,du2016/prometheus-docs,Cepave/openfalcon.io,chosenken/docs,prometheus/docs,ddfreyne/prometheus-docs,open-falcon/open-falcon.github.io,chosenken/docs,pdbogen/docs,du2016/prometheus-docs,ddfreyne/prometheus-docs,prometheus/docs,Cepave/openfalcon.io,chosenken/docs,ddfreyne/prometheus-docs,vic3lord/docs,tcolgate/docs,chosenken/docs,open-falcon/open-falcon.github.io,du2016/prometheus-docs,tcolgate/docs,tcolgate/docs