cloud: add tagging to metrics based on reason

Currently all metrics in cloud, but specifically the recently added `cloud.write_bytes` and `cloud.read_bytes` don't provide information about the reason why the cloud request was made. We use cloud storage for a large number of reasons in the code, and it could be valuable to have this breakdown.From a quick check, it appears there are a large number of these. A non-comprehensive list includes:
* Backup/Restore
* Import/Export
* CDC External Sink
* SQL Copy
* SQL Repair
* SQL File table external storage
* Disaggregated storage / Pebble Object reader
*
The best way to do this would to thread a reason through all the `ExternalStorage.*Factory` methods.

Jira issue: CRDB-42255