Hi all,
Recently I found the topic stats now have too many fields but some
might not help in certain cases, for example,
```
"offloadedStorageSize" : 0,
"lastOffloadLedgerId" : 0,
"lastOffloadSuccessTimeStamp" : 0,
"lastOffloadFailureTimeStamp" : 0,
"ongoingTxnCount" : 0,
"abortedTxnCount" : 0,
"committedTxnCount" : 0,
```
If offloader and transaction are not enabled, the fields above will
never help. There are also similar fields in other levels
(subscription, producer, consumer), for example, in subscription
level, we have following fields:
```
"allowOutOfOrderDelivery" : false,
"drainingHashesCount" : 0,
"drainingHashesClearedTotal" : 0,
"drainingHashesUnackedMessages" : 0,
```
The fields above are only meaningless for Key_Shared subscriptions, so
it could be a bit confusing if they're shown in other subscription
types.
I think we'd better adjust the hierarchy to make topic stats more
friendly, which might also reduce the memory usage. Though it might
have impacts on existing users because it will have breaking changes
on existing fields. WDYT?
Thanks,
Yunze
