You can view the metrics for a Queue on your account via the Cloudflare dashboard . Navigate to Workers > Queues > your Queue and under the Metrics tab you'll be able to view line charts describing the number of messages processed by final outcome, the number of messages in the backlog, and other important indicators.
we want to support sending messages via the rest api eventually, although it isn't on the roadmap right now. what's the use case you have in mind @mihaaai?