devserver: Support update payload metadata for full_payload artifact

BUG=chromium:403086
TEST=devserver_integration_test.py

Change-Id: Iad49f87c9ed79327f1eb5f4b03648f28110f4f15
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/1826048
Tested-by: Amin Hassani <ahassani@chromium.org>
Reviewed-by: Amin Hassani <ahassani@chromium.org>
Reviewed-by: Achuith Bhandarkar <achuith@chromium.org>
Commit-Queue: Amin Hassani <ahassani@chromium.org>
diff --git a/devserver_constants.py b/devserver_constants.py
index 70e8c2a..6f99cd0 100644
--- a/devserver_constants.py
+++ b/devserver_constants.py
@@ -46,6 +46,7 @@
 METADATA_HASH_FILE = 'metadata_hash'
 STATEFUL_FILE = 'stateful.tgz'
 UPDATE_FILE = 'update.gz'
+UPDATE_METADATA_FILE = 'update.gz.json'
 
 #### Android files
 ANDROID_BOOT_IMAGE_FILE = 'boot.img'