Add the backlog to the ledger of the topic's subscription

**Is your feature request related to a problem? Please describe.**
At present, when the disk space of bookie cannot be released, you can only view the topic subscription's backlog or the partition's backlog, but there are several ledgers in a partition. You don't know which ledger the backlog is stuck on.

**Describe the solution you'd like**
In the broker component, add a backlog on the ledger.

Start in the broker component where topic or partition subscription backlog is calculated, then trace how backlog state is associated with individual ledgers. Use the attached example to confirm the expected ledger-level information and add coverage for partitions containing multiple ledgers. Done means users can identify which ledger is retaining backlog.

Written by the indexing model from the issue text.