runtime/pprof: document labels bug

Currently only CPU profile utilizes tag information.
This change documents that fact

src/runtime/pprof/label.go

@@ -54,6 +54,8 @@ func WithLabels(ctx context.Context, labels LabelSet) context.Context {
 // Labels takes an even number of strings representing key-value pairs
 // and makes a LabelSet containing them.
 // A label overwrites a prior label with the same key.
+// BUG(hyangah): Currently only CPU profile utilizes labels information..
+// See https://github.com/golang/go/issues/23458 for details.
 func Labels(args ...string) LabelSet {
 	if len(args)%2 != 0 {
 		panic("uneven number of arguments to pprof.Labels")