Should we make Bits implement Accountable

Bits are returned from a few public APIs, notably LeafReader.getLiveDocs(), and can be significant consumers of heap. Most implementations are actually some form of BitSet, which implements Accountable, and means that clients can call ramBytesUsed() on them to get an idea of how much space they are using. Should we push this interface down to the base class? From a quick glance it seems that it would be pretty simple to implement, but maybe there is a historical reason that it wasn't done.

Start by tracing LeafReader.getLiveDocs() and the Bits base interface, then inspect BitSet implementations and Accountable's ramBytesUsed() contract. Check whether all Bits implementations can provide meaningful heap accounting and review existing tests around live documents; done means the API decision is resolved and any required implementation and tests consistently expose memory usage.

Written by the indexing model from the issue text.