[broker] Add Metadata type + method signature
[broker] Add Metadata type + method signature

mgranberry/ttn,mgranberry/ttn,TheThingsNetwork/ttn,TheThingsNetwork/ttn,TheThingsNetwork/ttn,mgranberry/ttn,LoRaWanSoFa/ttn,jvanmalder/ttn,LoRaWanSoFa/ttn,jvanmalder/ttn,jvanmalder/ttn