fix(groupBy): reexporting the GroupedObservable type (#3556)

We are now exporting the GroupedObservable type. This also marked the constructor as deprecated, as it is really for internal use only, and subject to change at any time.

fixes #3551

src/index.ts

@@ -1,6 +1,7 @@
 /* Observable */
 export { Observable } from './internal/Observable';
 export { ConnectableObservable } from './internal/observable/ConnectableObservable';
+export { GroupedObservable } from './internal/operators/groupBy';
 export { Operator } from './internal/Operator';
 export { observable } from './internal/symbol/observable';
 

src/internal/operators/groupBy.ts

@@ -253,6 +253,7 @@ class GroupDurationSubscriber<K, T> extends Subscriber<T> {
  * @class GroupedObservable<K, T>
  */
 export class GroupedObservable<K, T> extends Observable<T> {
+  /** @deprecated Do not construct this type. Internal use only */
   constructor(public key: K,
               private groupSubject: Subject<T>,
               private refCountSubscription?: RefCountSubscription) {