refactor(rosetta-icp): refactor and augment Rosetta ICP metrics

[fsodre]

This PR:

• Centralizes metrics set up and logic into a separate rosetta_core::metrics module, which can later be used to emit metrics from ICRC-Rosetta.
• Injects a actix-web-prom wrapper into the REST server, which automatically creates http requests metrics and the /metrics endpoint.
• Introduces a few new custom metrics: ledger_sync_blocks_fetched_total and ledger_sync_blocks_fetch_retries_total, besides the ones automatically added by actix-web-prom

[mbjorkqvist]

Thanks for this observability work @fsodre! What's the easiest way to test this? Is it simply spinning up the local cluster and seeing that things still work and perhaps seeing some data in some more Grafana dashboard panels, or should I be looking at the /metrics endpoint of the Rosetta instance, or something else?

There seem to be some CI failures that are not only related to formatting, so I'll make another pass once CI is green, but so far LGTM!

[mbjorkqvist]

It's a bit unfortunate that this unmaintained dependency has to be pulled in. It seems to only be used on this line in the actix-web-prom library. I had a quick look and found another couple of libraries similar to actix-web-prom, but they seem to be worse choices than actix-web-prom in terms on active maintenance, so I don't have any suggestions for improvement here.