requestinfohandler: Log caller_id from request metadata

What does this MR do and why?

Rails forwards caller_id in gRPC metadata, but Gitaly did not read or log it. During incident INC-13016, responders could not attribute Gitaly traffic to a caller. They could not tell whether elevated traffic came from crawlers, background jobs, or real users.

This reads caller_id from the request metadata and adds it to the request log tags, next to the existing remote_ip, user_id, and username fields. It is a log field only, not a Prometheus label, so there is no metric cardinality risk.

For GraphQL requests the value becomes more useful once gitlab!250311 (merged) lands, which sets caller_id to the operation, for example graphql:getProjects.

Related

• Rails side that sends the field: gitlab!250312 (merged)
• Corrective action: gitlab#614721