plugin/metadata: tweak the docs a little (#2685)
plugin/metadata: tweak the docs a little (#2685)

Add a simpler example that returns a static string the for metadata.

Signed-off-by: Miek Gieben <00b357f94cf3a1145a32703fb42e2d80418ab806@miek.nl>
coredns/coredns,miekg/coredns,ii/coredns,johnbelamaric/coredns,ii/coredns,johnbelamaric/coredns,yongtang/coredns,miekg/coredns,chrisohaver/coredns