Dedupe profile comments when merging profiles. (#303)

profile/merge.go

 
 	var timeNanos, durationNanos, period int64
 	var comments []string
+	seenComments := map[string]bool{}
 	var defaultSampleType string
 	for _, s := range srcs {
 		if timeNanos == 0 || s.TimeNanos < timeNanos {
 		if period == 0 || period < s.Period {
 			period = s.Period
 		}
-		comments = append(comments, s.Comments...)
+		for _, c := range s.Comments {
+			if seen := seenComments[c]; !seen {
+				comments = append(comments, c)
+				seenComments[c] = true
+			}
+		}
 		if defaultSampleType == "" {
 			defaultSampleType = s.DefaultSampleType
 		}

profile/profile_test.go

 	// location should add up to 0).
 
 	prof := testProfile1.Copy()
+	prof.Comments = []string{"comment1"}
 	p1, err := Merge([]*Profile{prof, prof})
 	if err != nil {
 		t.Errorf("merge error: %v", err)
 	if err != nil {
 		t.Errorf("merge error: %v", err)
 	}
+	if got, want := len(prof.Comments), 1; got != want {
+		t.Errorf("len(prof.Comments) = %d, want %d", got, want)
+	}
 
 	// Use aggregation to merge locations at function granularity.
 	if err := prof.Aggregate(false, true, false, false, false); err != nil {