Set up the metric extraction sink in server creation
Set up the metric extraction sink in server creation

This ties all the changes together: Now metrics from SSF spans flow
directly (via go channels) into the metric worker, without having to
traverse any network at all!

stripe/veneur,gphat/veneur,evanj/veneur,evanj/veneur,gphat/veneur,evanj/veneur,gphat/veneur,stripe/veneur