code: static <T> Observer<T> wrap(Observer<T> downstream,
                                  List<RunnableInstrumenter> instrumentations) {
          return new RxInstrumentedObserver<>(downstream, instrumentations);
      }
docs: Wrap a observer.
  @param downstream The downstream observer
  @param instrumentations The instrumentations
  @param <T> The type
  @return The wrapped subscriber