plugin/health: clarify server label (#1707)
plugin/health: clarify server label (#1707)

Health overloaded metrics does not carry the server label. Explain why.
jremond/coredns,ii/coredns,jremond/coredns,johnbelamaric/coredns,johnbelamaric/coredns,miekg/coredns,chrisohaver/coredns,ii/coredns,yongtang/coredns,miekg/coredns,coredns/coredns