fix: proxy mongodb storage fields overspecified

It's unnecessary to specify all the fields that should be returned by
a mongodb query, because by default, it will return all the
fields. This patch makes it so that only the '_id' field is mentioned
specifically, and that's because we don't want it leaking into the
transport layer.

Change-Id: Ib59e551b0c85f2972de86baeaa2c7e65738a9a47
Closes-Bug: 1230390
Merge "fix: proxy mongodb storage fields overspecified"