Atlas OA may rely on running queries directly on the system.buckets.X collection to archive and delete buckets. Once buckets are fetched they want to use  $_internalUnpackBucket to extract the measurements.

More and more arguments will be added to $_internalUnpackBucket due to rewrite philosophies around being able to serialize an "AST" node for transport from mongos <-> mongod. We don't want to expose all the possible knobs of $_internalUnpackBucket as part of a stable API for OA's usecase.

It was agreed during a meeting with QO that we would provide a sugared alias to $_internalUnpackBucket that only exposes a minimal set of parameters in the parser for them to be able to unpack buckets for their archive format. That set of params should be just the metaField and timeField since they do not require measurement field projection functionality.

Other requirements for this alias include:

• Provide internal documentation for OA.
• It should have a stable api so we need to implement tests that will signal that we have broken the stage for OA.
• This should not function in API strict mode to prevent external users with api versioning access to this stage.