observer

type Event struct {
	// Automatic fields
	Sequence core.EventID // Auto-filled
	Time     time.Time    // Auto-filled

	// Type, Scope, Context
	Type    EventType       // All events
	Scope   core.ScopeID    // All events
	Context context.Context // core.FromContext() and scope.Active()

	// Arguments (type-specific)
	Attribute  core.KeyValue   // SET_ATTRIBUTE
	Attributes []core.KeyValue // SET_ATTRIBUTES, LOG_EVENT
	Mutator    core.Mutator    // SET_ATTRIBUTE
	Mutators   []core.Mutator  // SET_ATTRIBUTES
	Arguments  []interface{}   // LOGF_EVENT
	Recovered  interface{}     // FINISH_SPAN

	// Values
	String  string // START_SPAN, EVENT, ...
	Float64 float64
	Parent  core.ScopeID // START_SPAN
	Stats   []core.Measurement
	Stat    core.Measurement
}

type EventType int

type Observer interface {
	Observe(data Event)
}