Collect metrics through Actor wrapper?

[strohel]

If I look correctly, metrics are currently only collected in System::run_actor_select_loop() -> recv(addr.message_rx) -> msg => arm. That opens a possibility to collect the metrics solely inside Actor::handle(), without any explicit involvement of System. For code reuse, this can be implemented through some MeteredActor wrapper around actual Actor. To achieve the same ergonomics as the current state, a MeteredSystem (containing System) could be introduced that would wrap each passed actor during spawn() (arriving de-facto near the starting point).

Potential advantages:

• The "core API" would be free of this "specific" concern - who doesn't care about metrics wouldn't have to know about them. "Only pay for what you use".
• The difference between System and SystemHandle would cease to exist, and these two could be merged into a single type. This difference was a source of light confusion when I initially read the API docs and code.
• Metrics collection could be enabled only for some actors, not "all or none". (this is currently achievable by spawning multiple systems, with some slight resulting inconvenience).
• Would allow more elaborate metrics collection patterns, like recording them every 10 messages, etc.

Disadvantages:

• Too many wrappers could make things like reasoning, debugging, rustc errors more complicated. (I would expect that rustc is able to compile-out the abstractions for zero runtime overhead though).
• Not sure if this fits with future plans for metrics (related: Improve metrics collection and reporting #7).
• more..?

This is spiritually similar to #26 - I have somehow fallen for Actor wrappers. ;-)