What's the equivalent of NamespaceManager.GetQueue.MessageCount in the new NET Standard Microsoft.Azure.ServiceBus?

To provide an update:

This is now implemented and available under the Microsoft.Azure.ServiceBus.Management; namespace.

NamespaceManager is now called ManagementClient and has (roughly) the same endpoints available.

Here's the class itself as part of the pull request to merge it into the main repository.