collab: Make metrics_id required in LlmTokenClaims (#21771)

This PR makes the `metrics_id` field on the `LlmTokenClaims` required,
as we always have one in practice.

Release Notes:

- N/A

crates/collab/src/api/billing.rs

@@ -130,7 +130,7 @@ async fn update_billing_preferences(
 
     SnowflakeRow::new(
         "Spend Limit Updated",
-        Some(user.metrics_id),
+        user.metrics_id,
         user.admin,
         None,
         json!({