Follow-up from "Fix GraphQL API mergeUser is null when mergeWhenPipelineSucceeds is false"

The following discussion from !76043 (merged) should be addressed:

• @manojmj started a discussion: (+3 comments)

  So the REST API exposes only object.metrics&.merged_by but GraphQL now exposes either of object.metrics&.merged_by || object.merge_user, which means we aren't exposing the same data in both. I wonder if we could make both these endpoints expose the same data 🤔

  Code wise, the change looks good, so I will pass to @cablett for maintainer review 🚀