code: func Use(c context.Context) context.Context {
  	lock := sync.Mutex{}
  	data := []LogEntry{}
  	return logging.SetFactory(c, func(ic context.Context) logging.Logger {
  		return &MemLogger{&lock, &data, logging.GetFields(ic)}
  	})
  }
docs: // Use adds a memory backed Logger to Context, with concrete type
  // *MemLogger. Casting to the concrete type can be used to inspect the
  // log output after running a test case, for example.