[attu metrics] add _env_ to support prometheus configuration

Signed-off-by: min.tian <min.tian.cn@gmail.com>

client/public/env-config.js

@@ -2,4 +2,8 @@ window._env_ = {
   MILVUS_URL: '127.0.0.1:19530',
   HOST_URL: 'http://127.0.0.1:3000',
   IS_ELECTRON: '{{IS_ELECTRON}}',
+  WITH_PROMETHEUS: '',
+  PROMETHEUS_ADDRESS: '',
+  PROMETHEUS_INSTANCE_NAME: '',
+  PROMETHEUS_NAMESPACE: ''
 };