[Anchor] Expose metrics about Anker from maintenance daemon

[ruuda]

The maintenance daemon monitors the on-chain Solido instance and exposes some info as Prometheus metrics. We should make it monitor the Anker instance as well, and expose its metrics.

• stSOL balance of the reserve.
• bSOL supply.
• Deposit histogram (depends on [Anchor] Record statistics about deposits and withdrawals #450).
• Cumulative UST value of the proceeds so far? (Then we would have to store that too.)

[enriquefynn]

As discussed off-channel.
We need a metric to know the price Anker sold stSOL rewards to especially know if we're being sandwiched ^^.
That can be done with two variables: st_sol_total_swaped_rewards and ust_total_swapped_rewards.
At every epoch we can subtract the new variables with the older one to know the swap price.