feat: support instrument stream under TreeRoot

[sticnarf]

I want await-tree to track a request stream passed to tonic. But the root future of tonic is apparently not under our control.

let req_stream = async_stream! {
    while x {
        whatever_fut.instrument_await("y").await; // not under any tree
        yield z;
    }
};
tonic_client.stream_rpc(req_stream);

And it doesn't work to use a customized executor that wraps every future with a TreeRoot. (Flood of future polled in a different context as it was first polled and future is dropped in a different context as it was first poll).

So, the best we can do is create a TreeRoot for the stream:

tonic_client.stream_rpc(tree_root.instrument_stream(req_stream));

[BugenZhao]

Looks great!