code: public Blob updateBlob(String bucketName, String blobName) {
      // [START updateBlob]
      Map<String, String> newMetadata = new HashMap<>();
      newMetadata.put("key", "value");
      storage.update(BlobInfo.newBuilder(bucketName, blobName).setMetadata(null).build());
      Blob blob =
          storage.update(BlobInfo.newBuilder(bucketName, blobName).setMetadata(newMetadata).build());
      // [END updateBlob]
      return blob;
    }
docs: [VARIABLE "my_blob_name"]