Use pprof labels in flame graphs

[metalmatze]

We don't really do anything with pprof labels in the UI right now. We only use them to create unique stacktrace IDs. What should we do with them in the UI?

[brancz]

I don't think I've seen any other flamegraph implementations with label support. I think they are intended to be aggregated away. If we can come up with a useful way to visualize them though, I'm all for it!