Add some guidance on recording rules naming.
Add some guidance on recording rules naming.

Also cover how to correctly handle means when aggregating.

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