IndexAccessMethod::insertKeys implementation just sets numInserted to the size of the input, even if some of the keys in the input set resulted in a no-op.

SERVER-128783 necessitated knowing whether any actual inserts had happened, but opted for not modifying the existing output parameter to preserve existing behavior.

Some instances of where this might end up:

• index_builds.side_writes.inserted
• slow-query log lines (keysInserted)
• the profiler (system.profile.keysInserted)
• currentOp / $currentOp keysInserted
• index_builds.keys_inserted_from_scan

The goal of this ticket is to assess if we actually want those metrics to represent "attempted to insert" rather than "actually inserted", and thus if we can simplify to having a single output parameter.