Add user_id to Gitaly via gRPC metadata

What does this MR do and why?

For #232645 (closed)

At the time of writing, Gitaly does not have user_id information when performing RPCs. This can be demonstrated in Gitaly logs. This information was not easy to extract from application context before. After user_id was added to application context in !101218 (merged), we can now extract it. Gitaly handled the remote_ip field automatically (here). We don't need to touch Gitaly in this MR.

How to set up and validate locally

• Before

• After

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