[node] add prometheus queries for balance monitoring

[ghost]

Things I'd like to query:

• current on-chain balances for all supported assets
• current off-chain balances for all supported assets
• balances limbo in transfer apps

[kilrau]

Since we currently have a couple of collateralized channels are gone, we'd also need calls to force close such channels and free funds.

[erkarl]

I cleaned up OP to the most basic balances I'd like to monitor via node API.