Operation status metric is incorrect for the prepare queue

[daniel-savu]

operation_status labels are incorrect in the prepare queue. For instance, check:

• this visualization of the prepare queue. Some messages have a status of ReadyToSubmit
• the source code, based on which a message can only have a ReadyToSubmit when pushed to the submit queue. queue.push(op, None) does result in an operation being pushed to a queue without having its status changed, but whenever an operation fails at the submit step, it's send back to the prepare queue with a Retry status (source)
• the logs for one of the messages in the queue at the time, which confirm that when popped it only had status ReadyToSubmit when in the submit queue, and Retry(ErrorSubmitting) otherwise: https://cloudlogging.app.goo.gl/5AnNqYQaPhyanWu56