First class notion of runtimes about console HOT 3 OPEN

Somewhat related, the task_group crate provides another way of grouping tasks and it would be awesome if that could be visualized as well. (just something that came to mind when reading this issue, I haven't actually attempted to use Console with a project that uses that crate)

from console.

This is also going to be necessary in order to integrate with Tokio's runtime metrics API (tokio-rs/tokio#4073), cc @LucioFranco.

from console.

I imagine the wire format changes are going to be something like this:

• add a "runtime ID" to identify runtimes
• add a "watch runtimes" RPC that returns a stream of runtime stats updates, and a map of any newly-detected runtimes (similarly to how the tasks RPC works)
• add (optional) runtime ID fields to tasks, identifying which runtime the task is running on.

from console.