exhaustMapWithTrailing/debounceMap implementation

I'm just opening (and closing) this to add an option to #1777, which I found and spent some time trying to get the examples working but ultimately found them lacking. I'm sorry if this isn't welcome (perhaps adding a stackoverflow Q&A would be better?)

This appears to work for me, hopefully it will be helpful to others:

// Sometimes we want the throttling behavior of exhaustMap without the losing
// the last emission. This is helpful in situations where we want to only issue
// one request at a time, but if we are told to issue another while issuing the
// first, we respect that. Think of it as a nicer switchMap.
//
// input   -1--2--3--4-------->
// project -1xxxxxxx3xxxxxxx4-> (x is async/waiting)
//
// Originally started with this, but did not work:
// https://github.com/ReactiveX/rxjs/issues/1777#issuecomment-466267803
export function exhaustMapWithTrailing<T, R>(
  project: (value: T, index: number) => ObservableInput<R>
): OperatorFunction<T, R> {
  return (source): Observable<R> => {
    const release = new Subject()

    return source.pipe(
      throttle(() => release, {
        leading: true,
        trailing: true,
      }),
      // TODO: Upgrade to TypeScript 3.6.3 when available and remove the cast
      // https://github.com/microsoft/TypeScript/issues/33131
      exhaustMap((value, index) =>
        from(project(value, index)).pipe(
          finalize(() => {
            release.next()
          })
        )
      ) as OperatorFunction<T, R>
    )
  }
}

/cc @edusperoni @Dorus

Start by reading issue #1777 and the TypeScript exhaustMapWithTrailing implementation and examples included here. Verify the proposed behavior for trailing emissions and determine whether the request has a settled scope; done requires an agreed implementation or documentation outcome.