add span plugin support

[zeebo]

this adds a hook so that span creation and completion can be observed. this lets us more cheaply hook every span that monkit creates without having to use the trace observer system.

[egonelbre]

Probably not worth the optimization, but we can reuse the buffer when retrying, rather than allocating a completely new one.

[zeebo]

concurrent callers could stomp over each other.

[egonelbre]

I meant reusing the nextPlugins:

func (r *Registry) AddSpanPlugin(h SpanPlugin) {
	var nextPlugins []SpanPlugin
	for {
		pluginsPtr := r.plugins.Load()
		if pluginsPtr == nil {
			r.plugins.CompareAndSwap(nil, new([]SpanPlugin))
			continue
		}
		plugins := *pluginsPtr

		nextPlugins = slices.Grow(nextPlugins[:0], len(plugins)+1)
		copy(nextPlugins, plugins...)
		nextPlugins[len(nextPlugins)-1] = h

		if r.plugins.CompareAndSwap(pluginsPtr, &nextPlugins) {
			return
		}
	}
}

[zeebo]

oooh yeah that would be fine of course.