feat(grafana): add per-backend circuit breaker inflight and max panels

- Per-Backend Inflight: current inflight requests gauge
- Per-Backend Max Inflight: configured max limit
- Both filterable by backend template variable

Author: mahmut-Abi

deploy/observability/grafana/pingora-gateway-observability-dashboard.json

@@ -2049,6 +2049,62 @@
       ],
       "title": "HTTP/3 Enabled",
       "type": "stat"
+    },
+    {
+      "datasource": {
+        "type": "prometheus",
+        "uid": "${DS_PROMETHEUS}"
+      },
+      "fieldConfig": {
+        "defaults": {
+          "unit": "short"
+        }
+      },
+      "gridPos": {
+        "h": 8,
+        "w": 12,
+        "x": 0,
+        "y": 91
+      },
+      "id": 68,
+      "options": {},
+      "targets": [
+        {
+          "expr": "nantian_gateway_dataplane_http_circuit_breaker_backend_inflight_current{backend=~\"$backend\"}",
+          "legendFormat": "{{backend}}",
+          "refId": "A"
+        }
+      ],
+      "title": "Circuit Breaker — Per-Backend Inflight",
+      "type": "timeseries"
+    },
+    {
+      "datasource": {
+        "type": "prometheus",
+        "uid": "${DS_PROMETHEUS}"
+      },
+      "fieldConfig": {
+        "defaults": {
+          "unit": "short"
+        }
+      },
+      "gridPos": {
+        "h": 8,
+        "w": 12,
+        "x": 12,
+        "y": 91
+      },
+      "id": 69,
+      "options": {},
+      "targets": [
+        {
+          "expr": "nantian_gateway_dataplane_http_circuit_breaker_backend_max_inflight_requests",
+          "legendFormat": "max inflight",
+          "refId": "A"
+        }
+      ],
+      "title": "Circuit Breaker — Per-Backend Max Inflight",
+      "type": "timeseries"
     }
   ],
   "version": 0,