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/rpc.rs

@@ -4941,6 +4941,7 @@ async fn get_llm_api_token(
     }
     let token = LlmTokenClaims::create(
         user.id,
+        user.github_login.clone(),
         session.is_staff(),
         session.current_plan(db).await?,
         &session.app_state.config,