Docs: Add note on how to find Praefect primary Gitaly via Curl

doc/administration/gitaly/praefect.md

@@ -1445,3 +1445,16 @@ Here are common errors and potential causes:
   - **GRPC::Unavailable (14:all SubCons are in TransientFailure...)**
     - Praefect cannot reach one or more of its child Gitaly nodes. Try running
       the Praefect connection checker to diagnose.
+
+### Determine primary Gitaly node
+
+To determine the current primary Gitaly node for a specific Praefect node:
+
+- Use the `Shard Primary Election` [Grafana chart](#grafana) on the [`Gitlab Omnibus - Praefect` dashboard](https://gitlab.com/gitlab-org/grafana-dashboards/-/blob/master/omnibus/praefect.json).
+  This is recommended.
+- If you do not have Grafana set up, use the following command on each host of each
+  Praefect node: 
+
+  ```shell
+  curl localhost:9652/metrics | grep gitaly_praefect_primaries`
+  ```