Review duplication in fields logged from Rails

A few fields are logging duplicate information in the logs now that we log context information (brought up by Sean in https://gitlab.com/gitlab-org/labkit-ruby/-/merge_requests/49#note_478172611)

1. json.remote_ip -> json.meta.remote_ip
2. json.username (and json.user_id) -> json.meta.user (I think we should also log the user_id inside meta).
3. json.controller (and json.action) -> json.meta.caller_id

The context can be propagated across services, and the fields need to be defined in labkit, ensuring consistent naming across those services for logging. Which is why I think we should remove the fields outside of the meta namespace. But some of the fields might be used in other tooling.