pkg/util/log: there are no individual sink metrics for fluent log sink

**Is your feature request related to a problem? Please describe.**
If my CRDB node is outputting logs to multiple targets using different log sinks, I would like to see which sink is causing more reconnection issues or write errors.

**Describe the solution you'd like**
Currently there are only metrics like "log.fluent.sink.conn.attempts", there should also be "log.fluent.sink..conn.attempts".

**Describe alternatives you've considered**
Multiple sinks but I monitor metrics at another level (like fluent forwarder).

Jira issue: CRDB-52805