The Read Preference spec specifies that the `nearest` mode should interact with tag sets by using members that match the tag sets - see https://github.com/10gen/specifications/blob/master/source/driver-read-preferences.rst#how-modes-interact-with-tag-sets

The Ruby driver currently ignores tag sets for the implementation of `nearest` - see #select_near_pool https://github.com/mongodb/mongo-ruby-driver/blob/1.x-stable/lib/mongo/functional/read_preference.rb#L165