Add format filter documentation.
Add format filter documentation.

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