What's the best way to add duration metrics to RPC requests?

[LesnyRumcajs]

Hey, I'd like to add duration metrics to an existing RPC server (which is based mostly at one of the examples here, apparently https://github.com/ChainSafe/forest/blob/e8420e42ad7b453c3ec72bbf62bdac692e00bc44/src/rpc/mod.rs#L380).

A Prometheus server is already there. Now, what is unclear to me is where should I put the metric logic. The API is a bit, quoting the example, weird. Should it be put roughly before the RPC middleweare is created, e.g., timer.start() and move it to the session_close task? Will this report correct time?

At the end of the day, I'd like to have a bunch of metrics that show me:

1. How often a given RPC method is called (both HTTP and WS)
2. Average response time, classified by method names.