gateway: more metric documentation
gateway: more metric documentation

GitOrigin-RevId: e8f7e4086269955119bab020fbbe265b9ec72da4

netsec-ethz/scion,netsec-ethz/scion,Oncilla/scion,netsec-ethz/scion,Oncilla/scion,Oncilla/scion,netsec-ethz/scion,Oncilla/scion