chore: Update discovery artifacts (#1346)

## Discovery Artifact Change Summary:
datacatalogv1beta1[ [More details]](https://github.com/googleapis/google-api-python-client/commit/c26eace22d9c3488cdebab828f158e7fc998476e)

chore(classroom): update the api
feat(datacatalog): update the api
chore(metastore): update the api
chore(tpu): update the api
chore(docs): Add new discovery artifacts and reference documents
diff --git a/googleapiclient/discovery_cache/documents/metastore.v1alpha.json b/googleapiclient/discovery_cache/documents/metastore.v1alpha.json
index 7587786..f2d8655 100644
--- a/googleapiclient/discovery_cache/documents/metastore.v1alpha.json
+++ b/googleapiclient/discovery_cache/documents/metastore.v1alpha.json
@@ -899,7 +899,7 @@
       }
     }
   },
-  "revision": "20210413",
+  "revision": "20210503",
   "rootUrl": "https://metastore.googleapis.com/",
   "schemas": {
     "AuditConfig": {
@@ -1447,7 +1447,7 @@
       "id": "MetadataImport",
       "properties": {
         "createTime": {
-          "description": "Output only. The time when the metadata import was created.",
+          "description": "Output only. The time when the metadata import was started.",
           "format": "google-datetime",
           "readOnly": true,
           "type": "string"