Add Prometheus metrics for RPC endpoints

[LesnyRumcajs]

Issue summary

Each RPC endpoint hit should be reflected in Forest metrics.

At the end of the day, we want to have dashboards showing how often is a given RPC method called.

Ideally, all methods would go through a single method that increases the counter on its, e.g., RPC name label.

Other information and links

[ruseinov]

It looks like axum supports middleware. Here's an example: https://github.com/tokio-rs/axum/blob/main/examples/consume-body-in-extractor-or-middleware/src/main.rs, there probably is a better one. I'm happy to take this on.

[LesnyRumcajs]

Another metric that should be included in the RPC endpoints monitoring is duration, so that it's easily verifiable how long given endpoints take to process.

[lemmih]

Constant space quantiles: https://docs.rs/average/latest/average/struct.Quantile.html Can be used to report how long it took to execute 50% of calls, 80% of calls, 99% of calls, etc.

[LesnyRumcajs]

Constant space quantiles: https://docs.rs/average/latest/average/struct.Quantile.html Can be used to report how long it took to execute 50% of calls, 80% of calls, 99% of calls, etc.

@lemmih not sure if it's needed given, e.g., https://prometheus.io/docs/concepts/metric_types/#summary or some other in-built histogram. Some more investigation is needed in that regard.