cdc: clean up the NetMetrics injection

To ease backporting, the NetMetrics were added to the `AggMetrics` for changefeeds. They don't follow the same pattern as the other `sliMetrics` and should be in a different struct. This would require a larger change to the metrics to implement.

Ideally instead of passing a `metricsRecorderBuilder` which only contains a `metricsRecorder` (which is a `sliMetrics`), we would pass in an interface that could get both the `sliMetrics` as well as the `NetMetrics` for the connection.

Jira issue: CRDB-42015