Implement stats command

Problem/Motivation

When debugging a queue, sometimes its difficult, especially for some transports, to inspect the overall size of a queue. Especially for non SQL-like, or if CLI tools for the transport backend in question are not available.

Determining the size of the queue is also useful for autoscaling workers. For example, https://fly.io/docs/launch/autoscale-by-metric/

And programmatically retrieving the size (as an integer) simply becomes sm messenger:stats asynchronous --format=json | jq '.transports.asynchronous.count // empty' || exit 1

Proposed resolution

Implement the stats command, as provided by Messenger component.

Add basic integration tests.

Remaining tasks

Do it.

User interface changes

Only the `sm` CLI interface.

API changes

New services

Data model changes

None