Export metrics to do with the Ethereum event oracle

[james-chf]

We should export metrics via the Prometheus endpoint that could be used for Grafana dashboards and also for alerts. For the ledger oracle component specifically, it's important validator operators know if this is not working properly for whatever reason (e.g. their ledger is unable to reach the Ethereum RPC endpoint), as then they will stop voting on Ethereum events. We should expose at the very least

• a current_block_height (of the Ethereum RPC endpoint) timeseries
• rate, error, duration metrics

[Fraccaman]

We can do this by directly listening to smart contract events/functions. I think Namada shouldn't have anything to do with monitoring but it would be nice to have some external tool to have some metrics

[james-chf]

Have updated the issue desc, this is specifically metrics for the oracle component of namadan ledger i.e. the client inside the ledger that calls the Ethereum endpoint. If this component starts erroring for whatever reason, we may want to expose something that validators can alert on.

We do emit WARN logs that validators could grep on, though this would be less robust.

[sug0]

this might be useful https://lib.rs/crates/opentelemetry