It appears there's a couple places where isMaster requests are constructed:
1) https://github.com/mongodb/mongo-go-driver/blob/master/x/mongo/driver/topology/topology_options.go#L198
2) https://github.com/mongodb/mongo-go-driver/blob/master/x/mongo/driver/topology/server.go#L514

While both set "appname", the latter uses "s.cfg.appname", but afaict that value is never set (and I've confirmed from logs that client metadata is being received without "application"). This makes sense, since "appname" comes from https://github.com/mongodb/mongo-go-driver/blob/master/x/mongo/driver/topology/server_options.go#L24, but there's no "WithAppName(...) ServerOption" function.