Tracking Issue for setOnCloseHandler being Experimental

To resolve issue #5895 PR #8452 has been created that adds new `ServerCallStreamObserver.setOnFinishHandler(...)` method.
The handler is called by `Listener.onComplete` _when the call is **finished** correctly from the server's point of view: either `onCompleted()` or `onError(Throwable)` has been called, all the messages and trailing metadata have been put on the wire and the stream has been closed_.

Several names were proposed for the handler:
- `onCompleteHandler` : derives name from `Listener`'s method but causes confusion with `StreamObserver.onCompleted()`
- `onSuccessHandler` : my initial idea, yet also confusing as it can be called also after `StreamObserver.onError(...)`
- `onFinishHandler` : current approach, matches well the verb from method's javadoc
- `onFinalizeHandler` : would also probably do well

Comienza con el issue #5895 y el PR #8452, y luego revisa la API propuesta ServerCallStreamObserver.setOnFinishHandler(...) y su debate sobre nombres alternativos. Confirma la denominación final y el estado de la API experimental, y actualiza este issue de seguimiento solo si el trabajo enlazado deja pendiente alguna decisión o acción de seguimiento.

Escrito por el modelo de indexación a partir del texto del issue.