plugin/acl: document metrics in README (#3605)
plugin/acl: document metrics in README (#3605)

Signed-off-by: Erfan Besharat <0c18043d8d413d388558f2a00442df03c780c106@gmail.com>
miekg/coredns,chrisohaver/coredns,yongtang/coredns,johnbelamaric/coredns,coredns/coredns,miekg/coredns,johnbelamaric/coredns