Improve wording in metric types documentation.
Improve wording in metric types documentation.

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