add /metrics for prometheus exporter

[dcolley]

add support for /metrics url

the config file now has entries for:

    "Prometheus": {
      "Enabled": true,
      "UpdateIntervalSeconds": 60
    }

Example output:

# HELP ibp_geodns_member_override Member override status in the GeoDNS network
# TYPE ibp_geodns_member_override gauge
ibp_geodns_member_override{memberName="Amforc"} 0
ibp_geodns_member_override{memberName="Dwellir"} 0
ibp_geodns_member_override{memberName="Gatotech"} 0
ibp_geodns_member_override{memberName="Helikon"} 0
ibp_geodns_member_override{memberName="Luckyfriday"} 0
ibp_geodns_member_override{memberName="Metaspan"} 0
ibp_geodns_member_override{memberName="Polkadotters"} 0
ibp_geodns_member_override{memberName="Radiumblock"} 0
ibp_geodns_member_override{memberName="Rotko"} 0
ibp_geodns_member_override{memberName="Stakeplus"} 0
ibp_geodns_member_override{memberName="Turboflakes"} 0
# HELP ibp_geodns_member_ping Member ping status in the GeoDNS network
# TYPE ibp_geodns_member_ping gauge
ibp_geodns_member_ping{memberName="Amforc"} 1
ibp_geodns_member_ping{memberName="Dwellir"} 1
ibp_geodns_member_ping{memberName="Gatotech"} 1
ibp_geodns_member_ping{memberName="Helikon"} 1
ibp_geodns_member_ping{memberName="Luckyfriday"} 1
ibp_geodns_member_ping{memberName="Metaspan"} 1
ibp_geodns_member_ping{memberName="Polkadotters"} 1
ibp_geodns_member_ping{memberName="Radiumblock"} 1
ibp_geodns_member_ping{memberName="Rotko"} 1
ibp_geodns_member_ping{memberName="Stakeplus"} 1
ibp_geodns_member_ping{memberName="Turboflakes"} 1
# HELP ibp_geodns_service_up Service status for each member in the GeoDNS network
# TYPE ibp_geodns_service_up gauge
ibp_geodns_service_up{memberName="Amforc",serviceId="ajuna.dotters.network::ssl"} 1
ibp_geodns_service_up{memberName="Amforc",serviceId="ajuna.dotters.network::wss"} 1
ibp_geodns_service_up{memberName="Amforc",serviceId="ajuna.ibp.network::ssl"} 1
ibp_geodns_service_up{memberName="Amforc",serviceId="ajuna.ibp.network::wss"} 1
ibp_geodns_service_up{memberName="Amforc",serviceId="asset-hub-kusama.dotters.network::ssl"} 1
ibp_geodns_service_up{memberName="Amforc",serviceId="asset-hub-kusama.dotters.network::wss"} 1
ibp_geodns_service_up{memberName="Amforc",serviceId="asset-hub-kusama.ibp.network::ssl"} 1
ibp_geodns_service_up{memberName="Amforc",serviceId="asset-hub-kusama.ibp.network::wss"} 1
ibp_geodns_service_up{memberName="Amforc",serviceId="asset-hub-paseo.dotters.network::ssl"} 1
ibp_geodns_service_up{memberName="Amforc",serviceId="asset-hub-paseo.dotters.network::wss"} 1
ibp_geodns_service_up{memberName="Amforc",serviceId="asset-hub-paseo.ibp.network::ssl"} 1
ibp_geodns_service_up{memberName="Amforc",serviceId="asset-hub-paseo.ibp.network::wss"} 1
ibp_geodns_service_up{memberName="Amforc",serviceId="asset-hub-polkadot.dotters.network::ssl"} 1
ibp_geodns_service_up{memberName="Amforc",serviceId="asset-hub-polkadot.dotters.network::wss"} 1
ibp_geodns_service_up{memberName="Amforc",serviceId="asset-hub-polkadot.ibp.network::ssl"} 1
ibp_geodns_service_up{memberName="Amforc",serviceId="asset-hub-polkadot.ibp.network::wss"} 1
ibp_geodns_service_up{memberName="Amforc",serviceId="asset-hub-westend.dotters.network::ssl"} 1
ibp_geodns_service_up{memberName="Amforc",serviceId="asset-hub-westend.dotters.network::wss"} 1
ibp_geodns_service_up{memberName="Amforc",serviceId="asset-hub-westend.ibp.network::ssl"} 1
ibp_geodns_service_up{memberName="Amforc",serviceId="asset-hub-westend.ibp.network::wss"} 1
ibp_geodns_service_up{memberName="Amforc",serviceId="bifrost-polkadot.dotters.network::ssl"} 1
ibp_geodns_service_up{memberName="Amforc",serviceId="bifrost-polkadot.dotters.network::wss"} 1
ibp_geodns_service_up{memberName="Amforc",serviceId="bifrost-polkadot.ibp.network::ssl"} 1
ibp_geodns_service_up{memberName="Amforc",serviceId="bifrost-polkadot.ibp.network::wss"} 1
ibp_geodns_service_up{memberName="Amforc",serviceId="bridge-hub-kusama.dotters.network::ssl"} 1
ibp_geodns_service_up{memberName="Amforc",serviceId="bridge-hub-kusama.dotters.network::wss"} 1
ibp_geodns_service_up{memberName="Amforc",serviceId="bridge-hub-kusama.ibp.network::ssl"} 1
ibp_geodns_service_up{memberName="Amforc",serviceId="bridge-hub-kusama.ibp.network::wss"} 1
ibp_geodns_service_up{memberName="Amforc",serviceId="bridge-hub-paseo.dotters.network::ssl"} 1
ibp_geodns_service_up{memberName="Amforc",serviceId="bridge-hub-paseo.dotters.network::wss"} 1
ibp_geodns_service_up{memberName="Amforc",serviceId="bridge-hub-paseo.ibp.network::ssl"} 1
ibp_geodns_service_up{memberName="Amforc",serviceId="bridge-hub-paseo.ibp.network::wss"} 1
ibp_geodns_service_up{memberName="Amforc",serviceId="bridge-hub-polkadot.dotters.network::ssl"} 1