runtime/pprof: document labels bug

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

Updates #23458

Change-Id: Ic893e85f63af0da9100d8cba7d3328c294e8c810
GitHub-Last-Rev: be99a126296493b3085aa5ade91895b36fb1de73
GitHub-Pull-Request: golang/go#27198
Reviewed-on: https://go-review.googlesource.com/c/go/+/131275Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>

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.
+// Currently only CPU profile utilizes labels information.
+// See https://golang.org/issue/23458 for details.
 func Labels(args ...string) LabelSet {
 	if len(args)%2 != 0 {
 		panic("uneven number of arguments to pprof.Labels")