br: fix a PromQL syntax error (pingcap#60705)

close pingcap#60212

Author: BornChanger

br/metrics/grafana/br.json

@@ -2917,7 +2917,7 @@
         "options": [
 
         ],
-        "query": "label_values(tikv_engine_size_bytes{k8s_cluster=\"$k8s_cluster\", tidb_cluster)",
+        "query": "label_values(tikv_engine_size_bytes{k8s_cluster=\"$k8s_cluster\"}, tidb_cluster)",
         "refresh": 2,
         "regex": "",
         "sort": 1,