feat(bigtableadmin): update the API

#### bigtableadmin:v2

The following keys were added:
- schemas.MultiClusterRoutingUseAny.properties.clusterIds.description
- schemas.MultiClusterRoutingUseAny.properties.clusterIds.items.type
- schemas.MultiClusterRoutingUseAny.properties.clusterIds.type

discovery/bigtableadmin-v2.json

@@ -1803,7 +1803,7 @@
       }
     }
   },
-  "revision": "20210907",
+  "revision": "20210921",
   "rootUrl": "https://bigtableadmin.googleapis.com/",
   "schemas": {
     "AppProfile": {
@@ -2730,7 +2730,15 @@
     "MultiClusterRoutingUseAny": {
       "description": "Read/write requests are routed to the nearest cluster in the instance, and will fail over to the nearest cluster that is available in the event of transient errors or delays. Clusters in a region are considered equidistant. Choosing this option sacrifices read-your-writes consistency to improve availability.",
       "id": "MultiClusterRoutingUseAny",
-      "properties": {},
+      "properties": {
+        "clusterIds": {
+          "description": "The set of clusters to route to. The order is ignored; clusters will be tried in order of distance. If left empty, all clusters are eligible.",
+          "items": {
+            "type": "string"
+          },
+          "type": "array"
+        }
+      },
       "type": "object"
     },
     "Operation": {

src/apis/bigtableadmin/v2.ts

@@ -762,7 +762,12 @@ export namespace bigtableadmin_v2 {
   /**
    * Read/write requests are routed to the nearest cluster in the instance, and will fail over to the nearest cluster that is available in the event of transient errors or delays. Clusters in a region are considered equidistant. Choosing this option sacrifices read-your-writes consistency to improve availability.
    */
-  export interface Schema$MultiClusterRoutingUseAny {}
+  export interface Schema$MultiClusterRoutingUseAny {
+    /**
+     * The set of clusters to route to. The order is ignored; clusters will be tried in order of distance. If left empty, all clusters are eligible.
+     */
+    clusterIds?: string[] | null;
+  }
   /**
    * This resource represents a long-running operation that is the result of a network API call.
    */