Paul Loyd

https://github.com/elfo-rs/elfo/blob/master/elfo-core/src/request_table.rs#L85

OpenMetrics defines exemplars and prometheus has supported them already in the experimental mode.

`ctx.request(..).await` doesn't produce any metric for responses. Actually, we produce no special counters for all incoming messages, but it's ok for incoming via the mailbox metrics because there is `elfo_handling_time_seconds_count`.

We don't need to print panics inside actors, because they are handled by the supervisor. Or we need to print them using the logger.

For now, it returns only responses.