Replacement for `bookie_journal_JOURNAL_CB_QUEUE_SIZE`

I see that https://github.com/apache/bookkeeper/pull/3830 removed the `bookie_journal_JOURNAL_CB_QUEUE_SIZE` metric. That metric tracked the number of entries from the point they entered the journal's queue to just after they are synced to disk (assuming `journalSyncData=true`).

Possible alternatives:
* `bookie_journal_JOURNAL_QUEUE_SIZE` - measures the journal `queue` size. It is incremented when entries are added to the `queue` and decremented after it was added to the `forceWriteRequests` queue (that queue is async, so all we know is that it was added to the queue).
* `bookie_journal_JOURNAL_FORCE_WRITE_QUEUE_SIZE` - measures the size of the `forceWriteRequests` queue. It is decremented just before calling `fsync`.

I think the `bookie_journal_JOURNAL_QUEUE_SIZE` seems like a good replacement, but it's not exactly the same metric. Is there any interest in re-adding the metric?

@merlimat - since you removed the metric, do you have any guidance? Thanks!