Document __scheme__ & __metrics_path__ labels available in relabeling.
Document __scheme__ & __metrics_path__ labels available in relabeling.

Fixes #153

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