Move concurrency log to RPC request logs

From 2023-04-13: file-87 goserver apdex drop (gitlab-com/gl-infra/production#8713 - closed).

In the above and similar incidents, it's super helpful to be aware of the current concurrency, broken down by ip/user/repository. We had this type of logs long before (here). Unfortunately, they are dumped independently of the RPC request logs. That makes it's hard to filter.

The alternative solution is to convert them into fields of RPC request logs with FieldProducers. This way unifies them at the same location and allows us to filter and explore them at once.