Option to capture `tokio::task::id()`

[Yen]

At the moment pprof-rs is able to capture thread ids, but for async applications this is not that useful as a filtering mechanism. The equivalent for tokio would be tokio::task::id() which would help filtering out specific tasks as they are provisioned across multiple threads.

Having this option behind a feature flag would be great for us, but if pprof-rs does not want to pull in a dependency like tokio for this (even behind a feature flag) then the ability to extend what data is captured in some way via a custom callback could be an alternative.

Thanks!

[ti-chi-bot]

Welcome @Yen! It looks like this is your first issue to tikv/pprof-rs 🎉