Track network/conversion fees

[mkurapov]

Context

We want to track the network/conversion fees that occurred for a particular outgoing payment.
We should have a new metric payment_fees, that is incremented by the following:
outgoingPayment.sentAmount - outgoingPayment.receiveAmount. Both of the amounts need to be converted to a base currency, how we currently do for the transactions_amount metric.

PR:

#2831

[JoblersTune]

We are going to want to calculate the difference before sending it off.
We had a lot of discussion about trying to differentiate between network fees vs. conversion fees but landed on that being impractical.
Also discussed issue where sender knows currencies but receiver knows final fees.
Set fees (by the ASE) are not included in this metric and are considered more private.
Issue to consider about additional 'fees' coming in when we convert to base currency.

[JoblersTune]

We'll need to add this info to Grafana for visualisation as well.

[koekiebox]

Adding link to comment made regarding preservePrivacy:
#2807 (comment)