实现 K8s client-go 的监控

// 用来将用户定义的监控采集实现注册到这里的全局变量，限制为只注册一次
var registerMetrics sync.Once

// LatencyMetric observes client latency partitioned by verb and url.
type LatencyMetric interface {
    // verb是指：PUT/DELETE/GET等
	Observe(verb string, u url.URL, latency time.Duration)
}

// ResultMetric counts response codes partitioned by method and host.
type ResultMetric interface {
    // 这里的method也是verb
	Increment(code string, method string, host string)
}

var (
    //组件内置的全局变量，用来接收用户的注册，组件会调用这些变量去采集数据。
    //如果用户不注册，这里的noop实现不会做任何事情
	// RequestLatency is the latency metric that rest clients will update.
	RequestLatency LatencyMetric = noopLatency{}
	// RequestResult is the result metric that rest clients will update.
	RequestResult ResultMetric = noopResult{}
)

// 注册监控实现，只能被注册两次
func Register(lm LatencyMetric, rm ResultMetric) {
	registerMetrics.Do(func() {
		RequestLatency = lm
		RequestResult = rm
	})
}

// request connects to the server and invokes the provided function when a server response is
// received. It handles retry behavior and up front validation of requests. It will invoke
// fn at most once. It will return an error if a problem occurred prior to connecting to the
// server - the provided function is responsible for handling server errors.
func (r *Request) request(fn func(*http.Request, *http.Response)) error {
	//Metrics for total request latency
	start := time.Now()
	defer func() {
		metrics.RequestLatency.Observe(r.verb, r.finalURLTemplate(), time.Since(start))
    }()
    
    // 省去其他代码
    // ...

var (
	requestLatency = prometheus.NewHistogramVec(
		prometheus.HistogramOpts{
			Name:    "name",
			Help:    "help",
			Buckets: prometheus.ExponentialBuckets(0.1, 1, 10),
		},
		[]string{"labelName", "verb", "url"},
	)
)

func init() {
	prometheus.MustRegister(requestLatency)
	metrics.Register(&metricsAdapter{requestLatency}, &resultAdapter{})
}

type metricsAdapter struct {
	m *prometheus.HistogramVec
}

func (l *metricsAdapter) Observe(verb string, u url.URL, latency time.Duration) {
	l.m.WithLabelValues("labelName", verb, u.String()).Observe(latency.Seconds())
}