Index builds have both a concept of a "protocol" and a concept of a "method".

The protocol defines how the index build is replicated; a single-phase index build is replicated via a single "createIndexes" oplog entry, a two-phase index build is replicated via "startIndexBuild" and "commitIndexBuild" oplog entries, and primary-driven index builds are replicated via "startIndexBuild", "commitIndexBuild", and ci/cd oplog entries.

The method defines whether the index build blocks concurrent writes or not; a foreground index build requires exclusive write access to the collection while a hybrid index build allows for concurrent writes.

Currently "primary-driven" is defined in the code as a method, despite it conceptually much more closely aligning with protocol. Thus, we should see if we can change it to be a protocol.