collab: Attach GitHub login to LLM spans (#16316)

This PR updates the LLM service to include the GitHub login on its
spans.

We need to pass this information through on the LLM token, so it will
temporarily be `None` until this change is deployed and new tokens have
been issued.

Release Notes:

- N/A

crates/collab/src/main.rs

@@ -151,6 +151,7 @@ async fn main() -> Result<()> {
                             method = ?request.method(),
                             matched_path,
                             user_id = tracing::field::Empty,
+                            login = tracing::field::Empty,
                             authn.jti = tracing::field::Empty
                         )
                     })