metrics: updated documentation
metrics: updated documentation

dongsupark/fleet,dalbani/fleet,coreos/fleet,endocode/fleet,jonboulle/fleet,dalbani/fleet,coreos/fleet,endocode/fleet,giantswarm/fleet,giantswarm/fleet,coreos/fleet,giantswarm/fleet,dalbani/fleet,dongsupark/fleet,jonboulle/fleet,dongsupark/fleet,jonboulle/fleet,endocode/fleet