Add global metric list including graphQL query

api/schema.graphqls

@@ -178,6 +178,18 @@ type NodeMetrics {
   metrics:    [JobMetricWithName!]!
 }
 
+type ClusterSupport {
+  cluster: String!
+  subClusters: [String!]!
+}
+
+type GlobalMetricListItem {
+  name: String!
+  unit: Unit!
+  scope: MetricScope!
+  availability: [ClusterSupport!]!
+}
+
 type Count {
   name:  String!
   count: Int!
@@ -192,6 +204,7 @@ type User {
 type Query {
   clusters:     [Cluster!]!   # List of all clusters
   tags:         [Tag!]!       # List of all tags
+  globalMetrics:   [GlobalMetricListItem!]!
 
   user(username: String!): User
   allocatedNodes(cluster: String!): [Count!]!