Gracefully ending a component-in-progress, e.g. batching metrics

[alexcrichton]

Currently the component model does not have any mechanisms by which the background work of a task, post return, can be gracefully cancelled or ended. A use case for this would be to have a wasi:http/service component which batches a number of requests together in terms of metrics and sends them off at the end. In this situation there's a tension when a host decides to stop sending requests to an instance and when the automatic batch-sending-logic will trigger. Ideally there would be some sort of signal that a host could send to a component and say "hey please shut down everything now" which post-return tasks would receive and could, for example, trigger a batched send. The host could then presumably place a hard deadline after this signal were sent to when the entire component is dropped anyway, but it'd have an opportunity to gracefully shut down in a sense.

This is spun out of #608 as the conclusion there is we probably won't be changing anything for the time being. This issue has historically been solved with a "stop" function of some form (or notification/etc) but hasn't been fully fleshed out yet (unless @lukewagner has a secret slide deck).

In any case I wanted to open an issue for this use case to have discussion about this possible direction.