[dnsdist] identical metrics

[Klaus Darilion]

Hi!

For performance reasons (it helps a lot) I have 3 listeners:
setLocal("0.0.0.0:53", { doTCP=true, reusePort=true, tcpFastOpenSize=100 })
addLocal("0.0.0.0:53", { doTCP=true, reusePort=true, tcpFastOpenSize=100 })
addLocal("0.0.0.0:53", { doTCP=true, reusePort=true, tcpFastOpenSize=100 })


/metrics then uses the same metric name:
dnsdist_frontend_queries{frontend="0.0.0.0:53",proto="udp"} 3459
dnsdist_frontend_queries{frontend="0.0.0.0:53",proto="udp"} 3516
dnsdist_frontend_queries{frontend="0.0.0.0:53",proto="udp"} 3422

I think this may cause problems with later visualization tools. I think
it would be good to add an optional "name" parameter (as with newServer).

regards
Klaus