Fix applying the trace context on events (#888)

Co-authored-by: Emir Ribić <[email protected]>

scope.go

@@ -383,6 +383,15 @@ func (scope *Scope) ApplyToEvent(event *Event, hint *EventHint, client *Client)
 		}
 
 		for key, value := range scope.contexts {
+			if key == "trace" && event.Type == transactionType {
+				// Do not override trace context of
+				// transactions, otherwise it breaks the
+				// transaction event representation.
+				// For error events, the trace context is used
+				// to link errors and traces/spans in Sentry.
+				continue
+			}
+
 			// Ensure we are not overwriting event fields
 			if _, ok := event.Contexts[key]; !ok {
 				event.Contexts[key] = cloneContext(value)
@@ -395,7 +404,9 @@ func (scope *Scope) ApplyToEvent(event *Event, hint *EventHint, client *Client)
 	}
 
 	if scope.span != nil {
-		event.Contexts["trace"] = scope.span.traceContext().Map()
+		if _, ok := event.Contexts["trace"]; !ok {
+			event.Contexts["trace"] = scope.span.traceContext().Map()
+		}
 
 		transaction := scope.span.GetTransaction()
 		if transaction != nil {

tracing.go

@@ -554,10 +554,11 @@ func (s *Span) toEvent() *Event {
 		s.dynamicSamplingContext = DynamicSamplingContextFromTransaction(s)
 	}
 
-	contexts := make(map[string]Context, len(s.contexts))
+	contexts := make(map[string]Context, len(s.contexts)+1)
 	for k, v := range s.contexts {
 		contexts[k] = cloneContext(v)
 	}
+	contexts["trace"] = s.traceContext().Map()
 
 	// Make sure that the transaction source is valid
 	transactionSource := s.Source