code: public Blob updateBlobWithMetageneration(String bucketName, String blobName) {
      // [START updateBlobWithMetageneration]
      Blob blob = storage.get(bucketName, blobName);
      BlobInfo updatedInfo = blob.toBuilder().setContentType("text/plain").build();
      storage.update(updatedInfo, BlobTargetOption.metagenerationMatch());
      // [END updateBlobWithMetageneration]
      return blob;
    }
docs: [VARIABLE "my_blob_name"]