Use external head for subgraph deployment alerts

stack_orchestrator/data/config/monitoring/subgraph-alert-rules.yml

@@ -19,7 +19,7 @@ groups:
                   type: prometheus
                   uid: PBFA97CFB590B2093
               editorMode: code
-              expr: ethereum_chain_head_number - on(network) group_right deployment_head{deployment=~"REPLACE_WITH_SUBGRAPH_IDS"}
+              expr: label_replace(latest_block_number{instance="external"}, "network", "$1", "chain", "(.*)") - on(network) group_right deployment_head{deployment=~"REPLACE_WITH_SUBGRAPH_IDS"}
               instant: true
               intervalMs: 1000
               legendFormat: __auto