changefeedccl: track and log per-changefeed metrics

One of the limitations of #121697 if it were to be implemented right now is that it would be limited to per-changefeed-label granularity. This means that the usefulness of a set of metrics for diagnosing problems with a specific changefeed is inversely proportional to how many changefeeds use that label. The default metrics label in particular is especially likely to correspond a high number of changefeeds.

We could update our metrics infrastructure so that we know the per-changefeed breakdown of metrics. (We still won't emit these for cardinality reasons.) Then we could periodically log the per-changefeed metrics for some subset of interesting changefeeds (e.g. top K changefeeds lagging by at least X).

Jira issue: CRDB-47176