Emitter: Try an event logger-style interface

[abhinav]

Instead of these several scoped objects,
we should maybe consider something like fxevent.Logger
where we have a single interface that accepts event objects,
and we have several kinds of event objects.
I'm not certain that this is desirable, but it's worth exploring.

[abhinav]

Quick update on this:
I've verified that it's possible to minimize the emitter interface to:

type EventLogger interface {
	LogEvent(context.Context, Event)
}

type Event interface{ event() }

With the following events,

type FlowStopEvent struct {
	Name    string
	Runtime time.Duration
	Err     error // if any
}

type ParallelStopEvent struct {
	Name    string
	Runtime time.Duration
	Err     error // if any
}

type TaskStopEvent struct {
	Name       string
	Runtime    time.Duration // zero if skipped
	ParentType string
	ParentName string
	Err        error // if any
	PanicValue any   // if any
	Recovered  bool  // recovered from panic/err
	Skipped    bool
}

type SchedulerStateEvent struct {
	DirectiveType string
	DirectiveName string
	State         scheduler.State
}

And still implement the metrics and log emitters we need with similar efficiency.
(There may be some room for more cleanup in the events.)

So I'm going to opt to delete the current emitter implementations (#2) for now,
and if we have time, we can replace the interface before initial release.
Otherwise, we can still do it immediately after.

[gcmurphy]

Is this still being worked on?