Replacement for `bookie_journal_JOURNAL_CB_QUEUE_SIZE`

[michaeljmarshall]

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!