expose wallet metrics

[cirocosta]

at the moment, monero-exporter has been solely concerned with exposing as prometheus metrics information extracted from a daemon's RPC endpoints.

that so far has been quite useful for my use-case, but when running something like https://github.com/comit-network/xmr-btc-swap ASB's, it becomes quite clear the need of also observing wallet-rpc.

e.g., see comit-network/xmr-btc-swap#652

given that cirocosta/go-monero, the module used for making those requests, already started including wallet procedure calls, we could optionally support wallet-rpc too.

[cirocosta]

ps.: as mentioned in that COMIT issue, it might even be worth implementing a collector that exposes metrics out of the logs it can parse from the daemon/wallet-rpc as some of that information might be useful aside from what the RPC endpoints give you (although I'd try to not rely on that as much). e.g., see https://github.com/cirocosta/dmesg_exporter as an example of exposing metrics out of a log buffer