Move pack-objects cache logs to RPC request logs

From 2023-04-13: file-87 goserver apdex drop (gitlab-com/gl-infra/production#8713 - closed), we noticed that pack-objects caching dumps out some logs. Those logs output directly using logrus. We can filter those logs independently, but it's quite hard to incorporate them with other logs since they do not include enough fields like normal RPC logs.

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.