code: func (s *Session) SelectServers(tags ...bson.D) {
  	s.m.Lock()
  	s.queryConfig.op.serverTags = tags
  	s.m.Unlock()
  }
docs: // SelectServers restricts communication to servers configured with the
  // given tags. For example, the following statement restricts servers
  // used for reading operations to those with both tag "disk" set to
  // "ssd" and tag "rack" set to 1:
  //
  //     session.SelectSlaves(bson.D{{"disk", "ssd"}, {"rack", 1}})
  //
  // Multiple sets of tags may be provided, in which case the used server
  // must match all tags within any one set.
  //
  // If a connection was previously assigned to the session due to the
  // current session mode (see Session.SetMode), the tag selection will
  // only be enforced after the session is refreshed.
  //
  // Relevant documentation:
  //
  //     http://docs.mongodb.org/manual/tutorial/configure-replica-set-tag-sets
  //