Propagate user_id, username, and remote_ip to gitaly for end-to-end logging

We currently do not log user_id and username in gitaly directly. While it is possible to get this information by joining on correlation_id, it is not readily available.

I'm mostly looking at this as a need for the SRE team. Being able to directly perform aggregations on fields like user_id is essential for operating gitlab.com.

IMO we should propagate user_id and username as grpc metadata where possible. Client IP address may also be useful to have.

This can then be logged by gitaly directly without requiring joins. Most log analysis tools don't have good support for joining anyway, if any.

cc @andrewn