skydns: cache add help functions
skydns: cache add help functions

Add Size and Capacty as helper function so we can export these as
metrics.

ZeaLoVe/skydns,netf/skydns,AdoHe/skydns,vinhhrv/skydns,yonglehou/skydns,andrewrothstein/skydns,tuannvm/skydns,zhang0137/skydns,loveshell/skydns,nicescale/skydns,tonicbupt/skydns,weekface/skydns,no2key/skydns,hw-qiaolei/skydns,skynetservices/skydns