Discussion on Default Implementation for isDriverMetadataServiceAvailable

[ZhangJian He]

Hi, bookKeepers

I've reviewed the PR that introduces `isDriverMetadataServiceAvailable`. I
have concerns about providing a default implementation that returns a
constant value like true, it's not a default interface like this.

https://github.com/apache/bookkeeper/pull/4342#discussion_r1591761669

```
default E getFirstElement() {
    return getElements().get(0);
}

List<E> getElements() {
}
```

- First we don't guarantee/make ABI compatible between minor releases
- Second, People who implement metadata drivers should implement this
correctly, it may lead to unintended behavior if it's not properly
overridden by all implementations.

I'd suggest making this method abstract instead to avoid potential
misinterpretation.

Thanks
ZhangJian He
Twitter: shoothzj
Wechat: shoothzj