code: func NewWriterPool(maxRate int, maxBurst time.Duration) *WriterPool {
  	return &WriterPool{
  		maxRate:  maxRate,
  		maxBurst: maxBurst,
  		givenOut: make(map[ThrottlerWriter]struct{}),
  	}
  }
docs: // NewWriterPool creates a pool that ensures the writers it wraps will
  // respect an overall maxRate, with maxBurst resolution. The semantics
  // of the wrapped writers are the same as those of using a plain
  // ThrottledWriter.