code: func (c *expvarStatsClient) WithTags(tags ...string) StatsClient {
  	m := &expvar.Map{}
  	m.Init()
  	c.m.Set(strings.Join(tags, ","), m)
  
  	return &expvarStatsClient{
  		m:    m,
  		tags: unionStringSlice(c.tags, tags),
  	}
  }
docs: // WithTags returns a new client with additional tags appended.