collab: Attach user_id to LLM spans (#16311)

This PR updates the LLM service to attach the user ID to the spans.

Release Notes:

- N/A

crates/collab/src/main.rs

@@ -150,6 +150,7 @@ async fn main() -> Result<()> {
                             "http_request",
                             method = ?request.method(),
                             matched_path,
+                            user_id = tracing::field::Empty,
                             authn.jti = tracing::field::Empty
                         )
                     })