CancellationToken on typed hub client methods (SignalR request)

[kierenj]

I'm looking to have a loop which reads work from a Channel and writes to clients via typed hub methods.

I'll be doing something like the following on a long-running task:

while (!cancel.IsCancellationRequested)
{
  var signal = await _channel.ReadAsync(cancel);
  await _context.Clients.Groups(groupNames).ProcessSignal(signal); // can't pass "cancel" here
}

I'm not sure of the range of time/effort a hub client call might take: would it make sense to support a CancellationToken argument on typed client hub calls?

(Background: #8346 (comment))

[analogrelay]

We don't have the capacity for this in 3.0 but are still interested in it. Putting on the backlog.