Why: Atlas consumers need to separate server-owned work from customer workload in $queryStats, using {"key.isInternal": {$ne: true}}.

What:
* Keys get isInternal: true only when the opCtx carries the origin flag.
* The field is emitted only when true, so existing keys and keyHash values do not change.
* Definition: the work belongs to the server, regardless of who triggered it. Per-shard requests made for a user command stay unmarked, including the two-phase write without shard key.
* Jstests cover the user-command cases, and the README documents the stamp idiom.