Thomas de Zeeuw

This also relates to the other APIs such as `ActorGroup::join_all` (#472) and `test::join`, `test::join_many` and `test::join_all` (#470).

A possible implementation would be for `heph-inbox::Manager` to implement `fn set_exit_status(self, ExiStatus)`. Then a new `Sender::monitor` API similar to that of `Sender::join`, but it returns this `ExitStatus`.

Could maybe use something like https://github.com/Thomasdezeeuw/inbox/pull/32 for this.

This could be used by external crates to export to systems like [Prometheus](https://prometheus.io).

Also need metrics from `Initiator`s, e.g. the number of requests handled and in progress.

Some more toughs. An `Observable` which can be implemented by a types that can be observed. ```rust pub trait Observable { // This would need to implement serde::Deserialise or something....

Maybe support OpenTelemtry: https://opentelemetry.io/, https://github.com/open-telemetry/opentelemetry-specification.

See https://github.com/Thomasdezeeuw/heph/pull/484.

I've just found out that panics in the spawn actors using `test::try_spawn_local` (and related) don't actually cause a test to fail. That seems like a problem.

Maybe #476 can help here.