should we add a metrics to show the top storage in bookeeper

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pulsar_storage_size only shows The total storage size of the topics in this namespace owned by this broker (bytes).

sometimes, if the topic was not loaded by brokers, maybe producers and consumers do not connect with any broker.

but they produced the most messages. we want to know， which topic/namespace/ tenant use the most disk in bookkeeper

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maybe we can read the zookeeper of the broker to analyze the ledger ids of a topic， then read the leger info in the zookeeper of the bookkeeper and sum them up。

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Start by reviewing the existing pulsar_storage_size metric and the ZooKeeper metadata for broker topics and BookKeeper ledgers. Clarify whether the result should rank topics, namespaces, tenants, or all three, and define how ledger sizes are aggregated. Done means an agreed metric design that identifies the largest storage users.

Written by the indexing model from the issue text.