changefeedccl: add observability for changefeed catchup progress

When a changefeed is catching up after a long period of lag, it can be difficult to identify progress. We have `distsender.rangefeed.catchup_ranges` that shows the number of ranges in catch up mode, but completing ranges can be bursty depending on the number of writes that need to be emitted. `changefeed.emitted.messages` shows that a given changefeed is sending messages and thus making progress, but doesn't provide a sense of how many messages have been processed so far and how many more are expected before the catchup is complete. It would be useful to have a metric (or other observability) that indicates the progress of the catchup period.

Jira issue: CRDB-51182