Update /oauth/token to log user IP and called username

Based on the investigation in https://gitlab.com/gitlab-sirt/shared-incidents/incident_4326/-/issues/1+ we identified that the oauth/token endpoint does not consistency record the callers username and IP in standard json IP fields. This corrective action is to add the 2 into logged messages.

Ask from SIRT (@ballam) - Can we please include User-Agent details too?