Possible Error: PMGD Metadata for descriptor set may be stored even if descriptor set creation fails

[ifadams]

Describe the bug
This is based on code reading, so still needs some additional checks to verify.

Basically, when we add a new descriptor set, we do the metadata operations first (construct protobuf), if these succeed, we then proceed to create the descriptor set (construct response).

However, given that metadata is committed before we attempt to create the descriptor set, we may wind up in an inconsistent state as theres no easy way to roll back a metadata commit with the PMGD based handler.

To Reproduce

NEED TO VERIFY

Add two named descriptor sets with identical names and engines in separate transactions, the second add should should fail.

Then query for the all metadata affiliated with a descriptor set, and if we see multiple node responses we'll know this is an active bug.

Expected behavior

We should block metadata from being created if a descriptor set is already in existence, and ideally if descriptor set creation fails, however this may be tricky for the PMGD based handler.