[Broker] Provide a way to retrieve the real msgBacklog value

[sijie]

Motivation

Currently, the broker exposes msgBacklog to topic & subscription stats. However, this value is an estimated value by calculation based on numProduced and numConsumed seen by a broker since it starts. This value is an estimated value.

We can consider providing a way to retrieve the real msgBacklog value from the storage or providing a tool to force load the metric into topic & subscription stat.