Function with_observer_task_sync 

pub fn with_observer_task_sync<F, R>(observer: Arc<dyn Observer>, f: F) -> R
where
    F: FnOnce() -> R,

Synchronous sibling of with_observer_task used by Instrumented<F>::poll so a single poll can bind / unbind the per-task observer without requiring an await. Spec 13 § 3.