For a long time, the only way to obtain the Service Bus entity(Queues, Topics) active Message count, or the size of the entity, you had to either rely on the old metrics pipeline or on the NamespaceManager library. We just added support for these two metrics, in our new Metrics pipeline. So far, you can only access these newly emitted metrics via code. We are working on showing them in the portal too, in the near future. Additionally the count and size is only available for Queues and Topics but not Subscriptions at the moment. The only currently supported way, is to observe the metrics in 1 minute intervals and observing the total aggregation. Other ways of observing these two metrics will likely give you wrong values.
Please find a sample on how to access the metrics here.
Even without portal representation this way of accessing the metrics should be valuable for auto receive or send side scaling scenarios.
Please find the main metrics blog and a few other interesting posts below: