Expose additional HTTP metrics

[IvoGoman]

Preflight Checklist

• I agree to follow the Code of Conduct that this project adheres to.
• I have searched the issue tracker for an issue that matches the one I want to file, without success.

Problem Description

Currently dex exposes only http_requests_total{handler="", code="",method=""} as a metric. The method used to calculate these metrics also collect the Duration and Response size written. https://github.com/felixge/httpsnoop/blob/master/capture_metrics.go#L46-L82
These metrics are not exposed.

Proposed Solution

Additional histogram metrics:

• http_request_duration_seconds{handler="", code="", method=""}
• http_response_size_bytes{handler="", code="", method=""}.

Which can be added to the handler instrumentalization here:

dex/server/server.go

Lines 335 to 356 in 5c66c71

	instrumentHandlerCounter := func(_ string, handler http.Handler) http.HandlerFunc {
	return handler.ServeHTTP
	}
	if c.PrometheusRegistry != nil {
	requestCounter := prometheus.NewCounterVec(prometheus.CounterOpts{
	Name: "http_requests_total",
	Help: "Count of all HTTP requests.",
	}, []string{"handler", "code", "method"})
	err = c.PrometheusRegistry.Register(requestCounter)
	if err != nil {
	return nil, fmt.Errorf("server: Failed to register Prometheus HTTP metrics: %v", err)
	}
	instrumentHandlerCounter = func(handlerName string, handler http.Handler) http.HandlerFunc {
	return func(w http.ResponseWriter, r *http.Request) {
	m := httpsnoop.CaptureMetrics(handler, w, r)
	requestCounter.With(prometheus.Labels{"handler": handlerName, "code": strconv.Itoa(m.Code), "method": r.Method}).Inc()
	}
	}
	}

Since native histograms are not yet GA, there is the open question of which Buckets make sense for these metrics.

Alternatives Considered

No response

Additional Information

No response

[nabokihms]

@IvoGoman the idea seems nice. Would you like to go ahead and make the change?

[IvoGoman]

Yes, I will open a PR for this later this week.