chore: Enable Prometheus for staging task processor (#5317)

Author: khvn26

infrastructure/aws/staging/ecs-task-definition-task-processor.json

@@ -154,6 +154,10 @@
                 {
                     "name": "FLAGSMITH_ON_FLAGSMITH_SERVER_OFFLINE_MODE",
                     "value": "False"
+                },
+                {
+                    "name": "PROMETHEUS_ENABLED",
+                    "value": "True"
                 }
             ],
             "secrets": [
@@ -225,4 +229,4 @@
     ],
     "cpu": "1024",
     "memory": "2048"
-}
+}