Currently, shards have some logic which executes based on whether a request has originated from a router. To determine this, we typically use OperationShardingState::isComingFromRouter, which inspects if the OperationShardingState has DatabaseVersion or ShardVersion installed.

One shortcoming of this is that routers are not able to express a request originates from a router if it is not shardVersioned – this is the case for $mergeCursors, see SERVER-81344. Another shortcoming is that it relies on routers having expressed a shardVersion; this makes this mechanism not useful for detecting (incorrect) situations where a router might not be doing so.

This ticket is to consider creating a more explicit signaling method for routers to signal that a request originated from a router or routing operation.