Warn about time adjustment in tags

Time adjustment is confused to many people, as you can see in jaegertracing#961.

This change adds a warning in tags if we do any time adjustments,
so that it's at least clear that adjustments happened.

model/adjuster/clockskew.go

@@ -173,7 +173,14 @@ func (a *clockSkewAdjuster) calculateSkew(child *node, parent *node) time.Durati
 }
 
 func (a *clockSkewAdjuster) adjustTimestamps(n *node, skew clockSkew) {
+	if skew.delta == 0 {
+		return
+	}
+
 	n.span.StartTime = n.span.StartTime.Add(skew.delta)
+	n.span.Tags = append(n.span.Tags, model.String("time.adjustment.warning", fmt.Sprintf("Warning: this span's timestamps were adjusted by %v", skew.delta)))
+	n.span.Tags = append(n.span.Tags, model.String("time.adjustment.url", "https://github.com/jaegertracing/jaeger/issues/961"))
+
 	for i := range n.span.Logs {
 		n.span.Logs[i].Timestamp = n.span.Logs[i].Timestamp.Add(skew.delta)
 	}