Make sure onBaggage is called for trimmed Spans

and clarify the semantics.

span.go

@@ -136,6 +136,7 @@ func (s *spanImpl) SetBaggageItem(restrictedKey, val string) opentracing.Span {
 
 	s.Lock()
 	defer s.Unlock()
+	s.onBaggage(canonicalKey, val)
 	if s.trim() {
 		return s
 	}
@@ -144,7 +145,6 @@ func (s *spanImpl) SetBaggageItem(restrictedKey, val string) opentracing.Span {
 		s.raw.Baggage = make(map[string]string)
 	}
 	s.raw.Baggage[canonicalKey] = val
-	s.onBaggage(canonicalKey, val)
 	return s
 }
 

tracer.go

@@ -21,7 +21,8 @@ type Options struct {
 	ShouldSample func(int64) bool
 	// TrimUnsampledSpans turns potentially expensive operations on unsampled
 	// Spans into no-ops. More precisely, tags, baggage items, and log events
-	// are silently discarded.
+	// are silently discarded. If NewSpanEventListener is set, the callbacks
+	// will still fire in that case.
 	TrimUnsampledSpans bool
 	// Recorder receives Spans which have been finished.
 	Recorder SpanRecorder