[runtime] Rethink Observability

[patrick-ogrady]

Interleaving metrics with context has become quite fragile/tedious. After #3077 is merged, we should take a step to look at what we do for observability and see how we can make things more ergonomic.

[patrick-ogrady]

What ultimately complicated everything is the support of short-lived actors for epoching (context/metrics was much simpler when we assumed all actors were long-lived).

[patrick-ogrady]

At least as it pertains to metrics tracking (for scoped), we could add the invariant that context must be held wherever metrics are registered but there may not be a programmatic way to enforce this.