Metadata: use instance objects instead of dictionary

Commit 8ad28f924c7ff4c932bd280032cf7c8ffe5aa20a added object
support to the metadata service. We can now use an instance
object format instead of dictionary.

Change-Id: Ibe6a1e68019ad59cb57c2e406ca38fdabb8eedec
Merge "Metadata: use instance objects instead of dictionary"