Future: Deadlines and Cancelation patterns

[ktoso]

We'll eventually want to also define patterns for distributed deadlines with context.

We'd like to be able to use context to protect tail latencies of services, e.g. if a service call is already past its deadline, there's no reason to continue working on the downstream calls it caused.

Downstreams would need to protect their expensive calls using something like:

func expensiveOperation(context: BaggageContext) throws { 
  guard context.deadline else { // specific spelling not sure about yet, also because swift concurrency 
    throw DeadlineExceeded(context)
  }
}

We would like to ensure cancelation works well with whatever the swift concurrency story becomes and how it'd handle cancelation as well.

---

Composing and "sub deadlines" must be easy to compose, it should be easily possible to "my entire work should take 1 minute (until time T)" but each sub task must take no longer than a few seconds (until t1)" so we must allow composing deadlines like such.

The deadline should always take the "lower of the current deadline, and the passed in one".

---

If we'd ever get function wrappers, it could become @DeadlineRespecting (terrible name, we'd find a good one) that does this automatically.