src/main/java/io/reactivex/plugins/RxJavaPlugins.java
@SuppressWarnings("rawtypes")
    public static void setOnObservableSubscribe(
            @Nullable BiFunction<? super Observable, ? super Observer, ? extends Observer> onObservableSubscribe) {
        if (lockdown) {
            throw new IllegalStateException("Plugins can't be changed anymore");
        }
        RxJavaPlugins.onObservableSubscribe = onObservableSubscribe;
    }
@SuppressWarnings("rawtypes")
    public static void setOnObservableSubscribe(
            @Nullable BiFunction<? super Observable, ? super Observer, ? extends Observer> onObservableSubscribe) {
        if (lockdown) {
            throw new IllegalStateException("Plugins can't be changed anymore");
        }
        RxJavaPlugins.onObservableSubscribe = onObservableSubscribe;
    }
Sets the specific hook function.
@param onObservableSubscribe the hook function to set, null allowed