ledgerexporter: expose configured network as label in prometheus metrics

[jacekn]

What problem does your feature solve?

I'd like to be able to easily differentiate between testnet and pubnet ledgerexporter instances

What would you like to see?

I'd like to see the network setting exposed as label in all metrics.

What alternatives are there?

It's possible to label metrics by updating settings on prometheus side but it's much simpler if labels come from the exporter directly