fix: Update default LXD meta-data with user meta-data (#5584)

This was previously unnecessary because:

a. LXD automatically appends the user.meta-data key to default meta-data.
b. In the presence of duplicate keys, PyYAML uses the last key.

This change is the cloud-init part of a set of changes that will enable cloud-init
to avoid depending on undefined behavior. In the future LXD may stop
appending user-defined meta-data to its default meta-data. This change
makes cloud-init forward compatible to LXD for when that change is
implemented.

GH-5575

cloudinit/sources/DataSourceLXD.py

@@ -210,8 +210,8 @@ def _get_data(self) -> bool:
         config = self._crawled_metadata.get("config", {})
         user_metadata = config.get("user.meta-data", {})
         if user_metadata:
-            user_metadata = _raw_instance_data_to_dict(
-                "user.meta-data", user_metadata
+            self.metadata.update(
+                _raw_instance_data_to_dict("user.meta-data", user_metadata)
             )
         if "user-data" in self._crawled_metadata:
             self.userdata_raw = self._crawled_metadata["user-data"]