ServerSelectionMetadata is just a ReadPreferenceSetting + secondaryOk. Since secondaryOk is equivalent to ReadPreference SecondaryPreferred, it is simpler to translate it so we have a single source of truth. This is part of the OP_MSG spec which specifies using $readPreference rather than $ssm. The $ssm metadata field will still be used when communicating with 3.4 nodes over OP_COMMAND, but it will be translated at the border.

As part of this work, ReadPreferenceSetting will be attached to the OperationContext on mongos in the same way that it is in mongod.